With all the obvious stuff like more girls, more positions, and some progression mechanics having already been suggested, there is one particular mechanic I wanted to suggest that, while it may make sense from a gameplay standpoint, is based around a fetish that not everyone might enjoy - as in, based around oviposition.
If there will be proper progression so that you're not so omnipresent from start of the game, this idea might just fit. So basically, once in a while, after you fuck enough girls, one of the most used lockers becomes a "special" one, and chooses a specific target - it may either be a girl with a specific haircut/hair color/body type, or one of the "special" girls if these get introduced. Any other girls, when caught by it, will play the usual animation and it will go through like normal, but when you catch the girl the locker "needs", it plays out a specific oviposition animation - as in, the animation drags out a little longer, as the "main" tentacle not only cums, bu also lays eggs into the caught girl's womb before releasing her. She will obviously attract more attention (possibly due to ending up with a protruding pregnant-looking belly), making more girls run away from the lockers while she is walking off somewhere else, but she will eventually attempt to "infest" an "uninfested" locker, which causes her to, for a lack of better wording, lock herself into the aforementioned locker and lay the eggs there before exiting (complete with an option to watch if you're into that kinda stuff...), with the lockers where this happened eventually getting taken over by tentacles. If we're going to go with the idea of some girls eventually becoming more eager to get tentacled and the school getting more and more suspicious about the whole thing if one's not careful, then those "carrier" girls have the highest chance to keep going back for more tentacle loving, but might draw more suspicion to the tentacle infestation thing.
I hope I didn't end up alienating anyone with such a weird idea. Of course, it might be a less gameplay-mandatory option so as to not alienate those who do draw the line at certain things, but having some impregnation and/or oviposition moments thrown in would definitely make this game more interesting for some pervs like me.
Yeah...Sorry for a little too much information. In any case, the game looks hella promising, and I'm sure looking forward to more updates. And sorry for not being able to donate - my university scholarship payouts are a bit too low for that :(