Whenever underwear gets implemented it'd be cool to be able to buy/research a pair of vibrating panties, maybe with their own controls so you could either very slowly work your way to your orgasm or send yourself ( or the subject that's wearing them) rocketing over the edge. It would also be nice to be able to keep them going outside the lab, for some exhibitionism as you interact with your coworkers, desperately trying not to cum in your pants in front of them. The exhibition thing could also be applied to things like vibrating cock rings and clit vibes as well.

Just a thought to add a touch of light breath control element to the game over all. Maybe aphrodisiacs and such can be pumped through it and for limited breathplay having a timer for it. 

it would be great for the ability to love on one's footpaws, and as for the growth/macro bit, perhaps a grow/shrink gun, with whatever slight limits if needed

To have an option where you can choose if a character has only one or multiple parts like dicks or holes 

I have a fetish for balls. Sue me. Anyways, the ability to put a subject's pair of balls in your mouth, and gargle/slosh them around in your mouth. The way Fek makes (almost) everything jiggly, it'd be pretty hot. 

Pretty self explanatory, stimulate a subject by deeply "inspecting" their mouth with your hands, either before or after cuming inside 

having a mechanic where you can display more dominance (hair pulling, head pulling, tail pulling, spanking) or submissiveness (Not using your hands on subject and just moving your hips, playing with yourself) when you are penetrating the subject with either a toy or your genitalia...