Aside from story progression, a high/grade school pack is at the top of my list and its something that hasn't really been done. The biggest thing I would like to see from this option would be an active career option where you can go with your child/teen sim to school. I also remember in Sims 3 you could run a daycare so that could be interesting as well or to at least be able to send your toddler to daycare to learn and not just send them there when your sim goes to work or travels.
And I think Bands or cheer could easily fit in a high school pack.