I consider babies a lost cause by now to be honest and having a couple of new interactions or a changing station wouldn't really excite me at this point.
Teens as you say are teens for much longer than babies are babies. There are so many things that could be added to the game that will make teens more special. They already did some cool things with mood swings and phases in Parenthood and I am still hoping for a pack that is more focused on them. Maybe if we ever get a school GP kind of thing they could do a lot with teens.
A bit out of topic:
I would also like to see a bit of a revamp on children. I want them to play tag and be more childish, right now when they are with friends all they can do without an object is talk.
Also a bit of focus on elders would go a long way. I am playing at the moment a household that consists of 2 elder sisters and the adult daughter of one of them. They have 2 cats and are a delight to play actually. But I wish we had more activities for them, I would love them to be able to knit while watching TV for example.