The swinger idea actually had me thinking about something else that I hadn't really given much thought to before but might be worth exploring in an adult game.
I think most people in a relationship have certain lines that they wouldn't want crossed, but would they both draw those lines in the same place? This is true even for people just identifying with the MC in a game to some degree I think (as opposed to someone just thinking of themselves as a puppetmaster or spectator), but much moreso in a real life relationship.
Would we both fly off the handle at even a little flirting or is there a higher bar? If I made a list of every fantasy I have from the most innocent to the most potentially objectionable and she did the same, probably both of us would not agree to the "worst" stuff on the other's list from each of our perspectives. But there could be some overlap.
What if there's things on my list that she'd agree to swap for certain things on her list and vice versa? Some things wouldn't make the cut obviously, but others might. For instance in my case if she wanted to make out with her hot female BFF, that would be a pretty low bar for me. Basically, I'd demand to watch and that's all it would take to make me happy no matter how far she wanted to go with her friend. If there was some token contact for me as well whether crotch grab/handjob/BJ/whatever from either of them that would be nice as well but entirely optional. More likely that would be on my list rather than hers though.
Kind of a horse trading type thing. If you give me this thing on my list, I'll give you this and this from your list... Maybe she really gets off on the idea of making me jealous but I'm adamant about not letting her actually fuck any other guy. But maybe it's enough just to roleplay it with her pretending to cheat and me pretending to be some random guy (or someone specific like the mailman, or Brad Pitt, or whatever). I wouldn't necessarily get much out of that beyond knowing that she's really turned on while I fuck her, but what could I trade for it? Most likely she'd agree to roleplay some slutty maid/nurse/actress/whatever I wanted in exchange (or maybe something more personal like her sister).
Or maybe she's got a thing for both exhibitionism and jealousy and she'll let me have something on my list if we go to some club and I let some guys grope her in front of the whole club (which I really wouldn't like). Would that be enough for her to arrange a BJ from her BFF who she knows I have a bit of a thing for? Maybe. Especially if she also gets off on being jealous herself.
Maybe she's willing to give me the top item on my list (or something close to it), in exchange for me having to watch her give some dude a BJ. I definitely wouldn't like that but she's not actually getting fucked. Would I trade that if she arranged to have a couple of her hot friends eat each other out while I fucked both of them? I wouldn't let her fuck someone else but I might make that trade.
Maybe that last deal doesn't quite work. Maybe I won't agree to the BJ but I will agree to letting some dude eat her out if I got what I wanted. Maybe she won't let me fuck someone else but she will let me fuck her while she eats her friend out. That wouldn't work for me but it might for some people.
By the same token if her top item was something crazy like being a cum bucket in some sort of gangbang or something, there's just nothing she could trade that would entice me to ever make that trade so that would have to be crossed off the list. She could give me everything on my list and it wouldn't be enough for that. The same would probably be true for some things on my list.