Considering electrostimulation and medical roleplay (eg. urethral stimulation) have become so 'mainstream' that most sex shops explicitly have sections for them I find it really weird how little they seem to be represented within hentai games; although I do admit some aspect like needle play (a personal favorite) / staples, and catheters / saline injections (eg. scrotal) are probably a lot less mainstream - such a shame, considering how much fun they actually are, while being minimally dangerous if done responsibly. There are far more gruesome fetishes that don't seem to be bothering people too much...
Other things that come to mind are wax play, wrapping (mummification), pet play / hucow and yes, even proper rope bondage. And while most of the above can be seen (very) occasionally, most of the time it's just for a CG or two, just to signal "oooh, they are doing some REALLY kinky shit NOW!", just so a checkbox can be ticked ("yes it's in the game") - instead of getting proper attention like one character's consistent main theme or even *gasp shock horror* the theme of a whole game. Come on people, one girl randomly having sex in a karada rope harness just so they can say "look! BONDAGE!" is mere teasing for a fan, nothing but a ruined orgasm. And frankly, playing some of the few strictly-text-based games that venture into these largely uncharted waters is not much better either.
Finally, as has been touched on above already, one hardly ever sees these themes and BDSM / domination in general portrayed in a caring, mutually affectionate fashion - it seems to be generally assumed that unless the dom is a vicious sadistic power-crazed monster then it's not being done properly, which is utter bullshit on so many levels it's not even funny. Sure, some folks are in it to be humiliated to the extreme and being treated like dirt, but that is in no way hard-tied to any of these other kinks...