The only thing that I can think of would be more romantic interactions, here's a little list
- cuddling in bed
- make the "make out" interaction a bit more romantic
- also make the "kiss" interaction romantic
- holding hands when walking
- make both the "cuddling" and "cuddling while watching movie" (movie hangouts) interactions more romantic
- when two lovers are taking selfies, make it so that the selfie taken is cuter; for example, a sim is kissing their partner's cheek selfie
- having crushes, even crushes in University because not all people are willing to play with mods that add those features
This is just to name a few, but personally, these would really make gameplay more fun for me. If I can think of other things I'll probably come back to edit!!
