Not only would I absolute LOVE a generations-type back; including more game play for babies/children/teens/elders, a family tree with more depth, more family oriented social interactions between grandparents and grandchildren, I also want to see some different types of traits added. Specifically traits like "humble" (willing to help, uncomfortable around expensive items, etc), empathetic (others' emotions affect this sim, can calm down sad or angry sims), gullible (easily pranked, easily scammed), brave , I am sure there are several others to add to the list. I also want to see the memory system depth added (you can have memories in TS4 but, from my experience, they don't affect gameplay all that much, only mood) and also likes/dislikes and turn-ons/offs.
With a generations EP, it'd also be super cool for gameplay to have a social interaction of "ask advice". So a child or grandchild can ask a parent or grandparent for their advice for anything from a love interest to which university to attend (if at all) or which career to join. I realize that would be really involved, but how cool would that be? I suppose it could operate similar to the parents' action of "influence to...",
Other game play options that could be patched in could be painted nails (should have been in spa day?) but all the women in the family could get matching nails for an upcoming wedding, I definitely agree with canes (or walkers) for elders, also maybe traits like "forgetful" or "grumpy" specific to elders, earned through adulthood like how certain traits are earned through childhood.