What type of game mechanics are you looking for?
First up, thank you for you interest, and the time wasted reading my incoherent ramblings below. Anyway...
...that's a really hard question; made even harder by the mentioned lack of "reference" games I could point at and say "look, like that one!". It just seems that almost all games consider a bondage CG either a "bad end" or a reward scene for the player, rather than an actual game mechanic related thing. To be honest, if I had a crystal clear idea of what the specific mechanic should be like I might actually be trying to make a game based on it by now regardless of how ludicrously unfeasible that sounds to me. The closest thing I found so far was "
Bonds", although still not nearly as complex/deep/long as I would have preferred. And possibly even "
Off the Cuff", although that one's a weird one for sure.
Naturally, a game where I get to manually do the actual tying-up/rigging would be an obviously attractive option - but I'm not stupid enough to fail to recognize what kind of monumental effort it would be to implement arbitrary rope mechanics over a character model into a 3D game engine (which this would most likely have to be, unless someone gets amazingly creative with what can be done in 2D). Especially if done properly, where ropes actually affect and squeeze the model - I swear i'll SCREAM if I see one more 3D render where a bra circuit or a length of rope is just floating around the body...
So yeah, that's a moonshot of sorts - would be awesome but I'm not holding my breath. More realistically, I'd be quite comfortable with what can be done within the confines of a traditional VN - simply a number of choices that influence how a specific bondage session is playing out. Whether I'm the one controlling the action or the one getting tied up, I'd like to be involved in how the scene unfolds, step by step, in a way that far transcends some hypothetical "well you have two choices during the CG which get you four slight variations of much the same". Yes, that would still mean a daunting sized spiderweb of choices/actions/consequences/variations - I'd have to be an experienced VN author to judge what would be realistically possible here, and I am most certainly nothing of the sorts.
Nonetheless, I'd really enjoy getting to choose/influence the specific pose, the degree of its severity/complexity, getting to play with a variety of toys from the painfully mundane all the way to the really exotic stuff conoisseurs wave around - I'm not saying the game should include obsure stuff one can only see in a dark Berlin basement but if a reasonably well supplied fetish sex shop has a category for it and sells it then yes I'd like it featured in the game; if a fetish porn site has pics of it then I'd like it to be in the game. And yes that definitely means far more than a couple of generic nipple clamps and some fluffy handcuffs here or there.
In the end though, this is NOT about any specific game mechanic or fetish. Yes, I'd loathe seeing completely unrelated ones forced in just to pad play time (no "match three" to reveal sexy pic, please!!!), but at the end of the day this is about being involved in a fetish/bondage-centric relationship with one or several partners, with copious amounts of agency concerning a) the flow of each scene in particular and b) the evolution of the relationship in general*. What specific game mechanic enables that is less important; what IS important is that be it mere VN text choices or something much more dynamic, it needs to feel intrinsically related to the bondage/fetish theme and the evolution of the MC/LI relationship, rather than being an "evade zombies" platformer with a kinky flavor. Heck, I sure wouldn't mind an engaging story unfolding in the background either - but at the end of the day, EVERYTHING else would have to take a back seat compared to the bondage/fetish focus and NOT the other way around.
Anywhoo, thanks for reading all this ooof and take care...
* oh, and please lose the "crazy bitch" stereotype. Please! I mean I'm not opposed to some of those being in the game if you wish, but PLEASE include some kind/loving relationships as well; it's insane that everyone seems to think submissive players all want to be humiliated within the last inch of their worthless little lives, as opposed to JUST BE IN AN AFFECTIONATE RELATIONSHIP with a domme. Fucking ridiculous!
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is one of my favorite counter-examples - I'd love to play "Sunstone, the game" if there was a bit (lot) more of the actual "action" in it...