Well, if we're making a list, I'll play. These improvements top my list:
1) more "individual" vocal responses from the girls, Monica and Saiko shouldn't say the exact same phrases all the time.
2) import of the full stable (yes, all 4) girls from v2 into v3.5 fast sex, with again above, individual responses based on their personality type.
3) a virtual hand to interact with the girls, aka like Real Kanojo/Real Girlfriend, including pinch, rub, smack, slap, grab, fondle responses (yeah, that's a pipedream)
4) v2 stories playable with the better v3.5 engine, perhaps even expanded or branching toward the end, if they are having to be rebuilt from scratch.
5) no significant talk/work about v4 until the above are done-ish.
Not asking for much, eh?