I would love to see real kids and real babies which mean more chaos Sims 2 like. Kids could fight, run after each other, jump on beds, let their toys everywhere. They could not want to do their homework and cry for nothing.
Babies could be more than objects. They could be changed, put on a stroller or on a mat etc. We could feed him on the couch or a chair.
Elders could be great grand parents or very mad type of soured people. They could be nice to kids or very frightening. They could take them home for weekend or holidays. They could do some activity with them and teach them stuff.
We could see them walking together or sit on a bench in the street. They could join a retirement house and receive some money after they stopped their career.
I would love all generation to be very different and not centered on adult behaviors. Even teenager could be physically more different than adult. Make them smaller please and different.
For me this could be the extension that save the game. Honestly. Family is so important and it could be so so fun to play if it was more chaotic and more different between each age. So we can have fun whatever age we're playing.