Being able to create the parents and grandparents of new Sims without needing to have them as part of the played family would be great. Create some new NPC as parents/grandparents/great grandparents, or pick from the existing ones to fill that role.
Preschool would be nice. Children sports would also be great. Could be an after school activity, active or rabbit hole.
A school buss coming to take your kids to school would be fun.
Teen pregnancy, condom and contraception, birth preparation... Have no hope for that ever happening, even as a DLC. Only option are mods like Wonderful Whims and the other Whims...