In my opinion. I would like to see reworked elders and babies first. Unlike the toddlers I dont really need pre teens but I wouldnt mind getting them.
Child to teen is enough for me but pre teens would mean I have a life stage more with my children to enjoy before they age up to adults someday.
Also im prop a minority here but I would prioritized elders reworked instread of babies.
Or maybe not a rework but and addition. This could come as a SP for me. I want my elders to feel old. optional of course, not every elder is near dead, this would be kinda ageism. But I want my elders to walk with a walking cane and having the ability to crochet. I want some more clothing which my grandparents would have worn. Also I want them to look older. Right now there is nearly no optional diffrence between them and adults which I think sucks,.. ythey are the most boring life stage and cmon. we all let some of our elders die because we want to move on with another story, because they are just THERE at some point. but not as present. Those additions can ofcourse also apply to younger sims.
Right now it seems like every sim dies at the age of 58, but humans get older than that. And im missing those elder years. Its not a shame to grow old. thats natural. Its a kids game so they wont show all the negative part of growing old, I understand that but I think what we have right now is not enough. I want to have the options to differ my sims more. rigt now I can also play one style of elders. Thats what it feels to me.
On the other and babies, I would like to see the update, I think they are boring objects right now but compared to my nepfhews they behaved really much the same. Babies dont do stuff. They just are there. cry, eat, sleep, dipers, repeat.
I think they could improve them by freeing them from the crib like many said , and giving us some options like playing with them on the floor (while they laying on their blankets) with some of their toys. just animatiing them. I would also like going for a walk with them. (stroller)
Having some clothing options, like wraped up in blancets, jumpers, socks and hats.