I really enjoy when we both play a variety of multiple characters. It can be a straight up harem or not, and we certainly don't have to have all characters active in every scene, and we can start out with fewer and add to the cast over time. This is more for main characters, ones that are permanent members of the crew.

Preferred genders for main characters are, in no specific order: Female, Futanari/Hermaphrodite (Cock & Balls & No Vagina, Cock & Vagina & No Balls, Cock & Balls & Vagina), Femboy (Hung or Not, Muscular Body or Not, Feminine curves or not. Need that feminine face though). I'm open to trying cuntboys (think of the aforementioned femboys, just a vagina instead of cock and balls). Males are ok for NPC like, background, or secondary characters. They can even have intimate or sexual scenes with main characters, I'm just not fond of them as main characters themselves, sorry. I'm weird if you haven't noticed ;P. And if you'd like, we can use gendered titles, pronouns, and nouns for the opposite gender, or even come up with new sets for multiple genders!

Tops, Bottoms, Dominants, Switches, Submissives. Preference is a mix of characters that aren't completely one end or the other. I've found flexibility and mixed roles to be fun and hot. Dominants being topped by another dominant or submissives, or being a soft top. Submissives being switches with eachother, or being powerbottoms. Even roles being flipped at times. Doms out of their element by being on the bottom, subs getting a power rush from being in charge. With sub on sub action they can be pleasuring and doing bondage on eachother, working together and competing to please their dom, being tied up and fucked together, and getting into trouble and peril together. Some role arrangements are 1) I play a mostly dom character & subs/switches to your subs/switches, 2) We both play a dom/switch character & subs/switches, 3) All character are essentially switches. I may be ok with 4) You playing a dom switch character (and maybe subs/switches) to my subs/switches characters.

A general, free form kink for Alpha/Beta/Omega, Dominant/Submissive setups. This can be anything we would like to explain or justify things in our setting. Like society being more sexually open or liberal, or these relations and categorizations are a cultural thing. They are accepted as the way things are or should be and don't raise any eyebrows. Characters can have an attraction to eachother even if they wouldn't pair up otherwise. Even slavery and kidnapping can be legal in some cases.

A huge kink for me. Light, like tape around the ankles, wrists, knees, and elbows to moderate where zipties and handcuffs get in included, characters are well and tightly bound either in a position or to an object or fixture, to extreme where the position is painful, they're covered in restraints, or mummified and completely wrapped up and encased, someone went overboard with rope or tap, or locked up in place with multiple heavy duty restraints. Usage of toys and gear of all kinds. Mixing of pain and pleasure. Twisted games and relationships. Let's have fun describing how characters are bound up and struggle. Go wild! And if the rp involves powers, bondage can suppress those.

Of course, bondage and BDSM wouldn't be complete without gags. From simple cleave gags and over the mouth tape to muffle and distort speech, to stuffed mouths and ball gags to make intelligible speech difficult and screams for help hard to hear, to extreme where it's layer upon layer for when you don't want to hear more than angry grunts and tormented moans. And typing their gagged speech? Oh yes! If powers are involved, gagging can suppress those.

Encasement, Mummification Completely wrapped up, covered, or encased from neck or mouth to ankles, or even the head and feet included. Sensitive regions, like breasts, the ass or asshole, or genitalia can be left exposed sometimes for easy access, for focus and objectification, or as humiliation.
'Permanent Bondage' Not actually permanent or forever but it can certainly look or feel that way. Locked away for a long time or the risk of a long time. Solitary confinement, locked up and away, entombed, extreme and excessive restraints, and so on.
Objectification Characters being bound in ways to serve as objects or decorations
Self Bondage Characters doing bondage on themselves, either to please someone else, to try struggling out of for practice or fun, or for the thrill of being found or exposed. They can also do it so they can't get out without assistance, or maybe something went wrong and they're unable to free themselves.
Bondage by Other Means Things instead of characters being the ones that do the binding. Robots, creatures, machines, magical or enchanted objects and gear, etc. 

Thoughts for relations between characters. Like being loved, spoiled, abused, and humiliated by the same person. Abuse by or to loved ones is a sign of endearment right alongside normal displays of affection, and can be mixed together. Move night where they watch a horror porno after they've tied eachother up. BDSM themed slumber parties. Nights out bound, gagged, and exposed. Your brats, princesses, and in between. Princesses could be asking for BDSM as a reward for good behavior. "You /promised/ me you'd tie me up and stuff me with toys, Mistress!", "I don't think a cleave gag is enough, Mistress, others could still hear me screaming. You'd better stuff my mouth first, then tape over the cleave gag, Mistress please." For brats, misbehaving in order to get punished. "Yeah I broke into your office and fingered my ass on camera during your video meeting. It was a stupid boring meeting and you should have been playing with /me/ instead!...Nnff! These ropes are nothing. Hnnt! Y-you better make em tighter and cuff me too, if you don't want me getting loose and kicking your genitals in the next few minutes! A-and that gag is too small! I can easily get that out of my mouth! You suck as a dom…H-hey! W-what do you think you're gonna do with that thing?!" And inbetween, of course! Comfort and aftercare are just as desirable. Cuddling, massages, praise for quivering, sweaty, exhausted, cum soaked darlings after a rough session. And then there's whatever sexy torment and horrors that antagonists inflict. Maybe characters would need to be rebroken or rehabilitated once they're rescued. 

Holding characters on the edge of sweet release for hours or bringing them to a near climax before yanking stimulation away to force them to cool down. Forcing them to orgasm again and again past the point where it feels good, to where they dread the next aching, painful, tormenting dry climax. Maybe there's in setting reasons for doing so?

Impact Play Spanking, whipping, paddling, slapping, punching, being struck on sensitive parts of the body.
Electro Play Electricity and shocks generated by normal or magical means, applied to any part of the body by prods, clamps, clips, collars, etc.
Roughplay Tugging hair, rough groping, tossing, manhandling, etc.
Breathplay, Waterplay In place of, or addition to, more extreme or fantastical snuff like kinks. Smothering, hanging, choking, strangling, asphyxiation, drowning. Taking willing characters or forcing them right to the edge or beyond it as they struggle for air, lungs burning and bodies tingling as vision and consciousness fades. They can then be given a breather and subjected again, or pass out and wake up later.
Bladderplay, Enemaplay Sounding, catheters, stents, filling the bladder and/or ass with water, jello, viscous syrupy fluids, absorbing beads, etc. A note for this is that the bladder and ass will always be clean and any natural fluid will be non filth, like natural lube, slick, even cum.
Wetting Play Likewise only if the 'urine' is replaced with something else. This is for characters being unable to keep it together due to fear, pleasure, or were unable or forbidden from relieving themselves and could no longer hold the unrelenting building pressure back.
Cervix Play, Womb Play The cervix being used to cause pleasure and pain, or being to gain access to the womb. Stretching or filling the vagina or womb with cum, cocks, toys, fluids, inflatables, etc.
Peril Play Characters being placed in perilous situations, in bondage or not, that appear lethal. But instead of death it can be a loss of consciousness.
Medical Play Examinations, drugs, gas masks, experimentations, rubber, straightjackets, hospitals, asylums, etc.
Twisted Games Force characters to try making use of deprived senses, talk when gagged, manipulate or reach for objects and move when bound. Hunting and chasing. Failed escape attempts.

Having some characters related to eachother. Sisters, cousins, mothers/daughters, aunts/nieces. They can be brought into the rp already related, or be the aged up offspring between characters and added in later.

Sex, orgasms, edging, arousal, lust. Maybe there's something more to these in the setting. As a way to power machines, magic, rituals, etc. Either by how long they can last being edged, how many times they can cum, or a mix. Push them beyond their limits! In settings where characters have powers or abilities, usage can cause arousal. Use them too much without the willpower to control yourself and you'll end up a horny quivering mess covered in your own cum. Being edged or forced to orgasm against your will depletes your mana or suppresses your powers.

Sweat, milk, cum, slick, pheromones, all having sweet and pleasant scents and flavors. Sweet, fruity, simple, complex, all are great. Minty? Milk chocolatey? Blends? Go for it! Sweat is great for wet, glistening skin and damp, clingy clothing. Bladders can be reservoirs for cum, squirting cum during orgasms. We can even have milk squirt out of nipples when climaxing, slick from the ass, etc. Characters can be milked to collect all these sweet nectars and milk. These can even have special properties, aside from being nutritious. Like magical ones, for use in rituals or components of potions, etc. An even further out there idea is ejaculate can be something different and completely unrealistic at times. Gold? Diamonds?

All sorts! Dildos, vibrators, strap ons, beads, sex and fucking machines that characters could be strapped into. Normal size, oversized, exoticly shaped, etc.

Drugs, chemicals, magic, gas, etc. Anything that could turn even the most prude and chaste into a sex crazed beast.

The ass will always be naturally squeaky clean and spotless. Anything from anal sex and rimming, to having it function somewhat like a self lubricating pussy, to being used as a womb or incubator. Even as debauched as having it serve as a beverage container of fruit punch bowl at a party.

The trend for clothing is to be tight or revealing or even like a second skin. Even reverse styles where privates are exposed instead of covered. Latex versions of clothing.

Wombs, even the ass, used to breed or incubate all manner of mutants and creatures. Even objects, like pearls.

A wide range spanning physical and mental changes. Ideas for physical changes are things like gender changes, Alpha/Beta/Omega changes. Are they given a heat cycle if they didn't have it before, or was it made stronger? Increased sensitivity. Was their body resculpted to be more attractive, given scents and flavors they didn't have before? For Sci Fi settings do they get physical augments or cyberization? Are they given abilities or powers, or enhanced to be damage resistant or the power to regenerate or revive from death? For the mental aspect, changing their beliefs, making them see their new existence as proper or positive. Bimbofication but not with IQ loss, it would be more them using their intelligence and skills to new ends to please their master and mates, furthering their goals. Someone who was a fashion designer could now design fetish clothing. These changes can be forced on them or maybe they desire the change and sought it out, and be done by science, magic, surgery, hypnosis, mental conditioning and mind fucking, and drugs and chemicals. We can play these out in detail or not, have it be several scenes or something short and fast.

Consensual Characters consent to what happens or desire it.
Dubious Consent Coercion, pressure, even blackmail. Both wanting or enjoying what happens in some ways and not in others.
Non Consensual into Consensual Characters being broken in and coming to think of their new like positively.
Mindscrew, Mindfuck Changing characters' beliefs over time, getting them to act in ways or believe things they wouldn't have before. Can be done sweetly and gently in order to help them heal or cope from events to more maliciously in order to have fun. An example, a cult victim brought into a clinic for treatment. The doctor instead encourages the patient's vices and convinces them that they would never make it on their own and that the doctor will love and care for them.
Twisted Courtship Normally abusive or predatory behaviors are seen in a positive light, as an indication the recipient is desirable, especially if the perpetrator is someone the victim could lust after. Like an alpha/omega, dom/sub pairing. Stalking, harassment, molestation, abuse, kidnapping, etc. The cute barista or sex clothing and toy shop salesgirl being hit on before snatched and spirited away to a new life. The cop getting hit on by a special agent then being rescued after cultists or a serial killer messes with her. The heiress of an estate calls for an exorcist after paranormal entities start haunting her only for the exorcist to start messing with her just as much.
Mindbreak Raunchy wild sex and kinks turning even the most proud and defiant into quivering, drooling, incoherent messes.
Pride & Humiliation Collars, marks of ownership, pet names, or maybe these are things of pride and affection, depending on the circumstance. Pick out characters' clothes for them, or make them strip. Take away permissions and privileges, like clothes and sex toys, or even pleasure and orgasms. Make them beg and plead and debase themselves in order to get off. Chain them up for public display and let others have fun with them.
Corruption The warping of characters' perceptions and priorities, either slowly or quickly, forcibly or willingly, subtly or overtly, such that the master and the harem are what matter most no matter what their previous life was. A proud warrior beaten and stripped. A faithful priestess defiled and taken from holy ground. A confident adventurer trapped and made to disappear. A cop or mother, perhaps along with her daughters, despoiled and abducted. An executive or politician humiliated and never seen again.

Kidnapping, abduction, slavery, rape, dehumanization. Permanent bondage, identity death, even fatalities. Permanent ends are more for secondary characters, done to impact main characters. Though if we have ways for main characters to be restored or recovered we can do these things to them as well.

Related to *Bad Ends* and more for main characters we don't want perishing or removed from the RP. Like taking little to no lasting damage and harm, or they recover, no matter what's inflicted on them. Extreme BDSM, abuse, torture. Even if snuffed out they can come back via regeneration, magical rituals, high tech body cloning and mind downloads, etc. Perhaps they even get a bit of a high from it? Either being dead leaves them in a fuzzy warm darkness, or they can be completely aware and still feel things. And while in this state there is a growing feeling of needing to cum but being unable to. Then when restored they can finally have their mind melting release. It can be terrifying or unwanted when forced on them by a baddy, or as punishment, but euphoric when done for fun or brat punishment. A corny example...

"Hey Mommy, where'd Sis go?"

"Remember when she hogtied me and locked me in the oven for a few hours? Well, she's chained in the pool with a weight around her neck for the day as punishment…if you dare free her, you're gonna get buried in cement for a week with your pussy sticking out…unless that's what you're going to beg me for."

I love tossing ideas back and forth to come up with things we'll both enjoy. Settings and genres, characters, events and adventures, etc. Explanations and reasons for why things are the way they are, why the normally impossible isn't impossible in our worlds. Playing by ear and creating as we go is good as well.

I love using images for RPs. As references for characters, scenes, clothing, objects, plot ideas, etc. Even things we find neat or erotic that are only tangentially related. And yes this includes the use of AI generated ones. I know some don't like AI, and if you really don't I'll refrain from using it, but I've found it easier to make decent looking characters references for myself and my rp partner.

Nothing complicated or super involved like Dungeons & Dragons, but some kind of dice and character stats system for determining outcomes. Combat, hacking, stealth, interrogation, resisting torture, how long it takes to orgasm and how many they can handle, or how long can they resist cumming, attempt to escape from restraints, etc.