To start with, the base game needs a few updates:
Cars! (This is a MAJOR oversight)
Non-Vegan food! (Is it really necessary to put an outdoor BBQ grill in the kitchen in order to eat real food? Who in the hell WANTS a Faux BLT?)
More music genre options (Where are the rock or metal options?)
A greater range of personality traits and aspirations
Several expansions are missing things that could go a LONG way towards improving the game:
Get to Work: More careers that aren't merely rabbit holes
Cats & Dogs: More veterinarian options (there is only one vet office)
Seasons: The game could benefit greatly from a greater range of holiday traditions to choose from
Get Together: A greater range of club activity options
Get Famous: Model or Rock Star career options would be nice
I think there are many other ideas that could help improve the game, but these base game options are major oversights that should have been handled long ago.