1) More holiday traditions and multicultural items such as muslim, jewish, buddhist, hindu, and indigenous customs
2) parrot pets (full fledged)
3) tweens/another life stage between kid and teenager
4) a museum career / stuff to do with museums, zoos, aquaria, etc., maybe a curator career or paleontologist career, that kind of thing
5) more musical instruments and creative hobbies!!! drums, where are drums??? we could have weaving! more developed sculpting options?
6) 90s retro stuff!! (also y2k, both would be perfect)
7) More activities and skills for kids, more aspirations for kids, maybe going to school with kids like we now can with teens, and that would all extend to tweens should they be added - just make the kid parts of life more interesting!
8) in that vein the adult stages should probably be broken up more to make it more interesting but that's a little more complicated and probably not possible before the game is done, but stuff to think about in the future I guess
9) disability stuff like wheelchairs and other mobility aids, hearing aids, stim toys, medications, etc. since that stuff is a part of life!!
10) complete gender customization so you can have your sim be whatever you want - more options, not fewer! stuff like identifying gender labels, "gender neutral" clothing, intersex diversity, and so on !!