I would love for each life stage to be more unique. I am not sure I feel the need for a tween lifestage, but definitely make teens (and to an extent young adults) "younger".
A small start would be bringing back the "hang out" option for teens, but also other new interactions. Also what is up with school not building skills? Now for me, this is what I would love to see:
* Unique aspirations for each life-stage
* Skills that are more complex, with some skills building on others and some specific to each life stage (like toddler and child skills)
* Interactions and activities that are unique for each life-stage
* Needs and relationships changing at different speeds at different stages of life (teens falling in and out of love faster, social need decaying faster in teens and young adults, embarrassed moodlets more common as teens and almost non-existent an elder. Bladder and energy need decaying faster in old age...)