Well, in terms of what I think the game needs:
- A Music/Celebrity pack. Just something that adds more instruments and the ability to form bands in the game.
- Seasons. Duh.
- Witches. Of course.
- Werewolves. Why not?
- A Danger pack. Consequences and difficulty are lacking in TS4. I think a pack dedicated to making your Sims a bit more difficult to take care of would be great. It could also add a Fear emotion, something that would be perfect for the game and is oddly missing.
- Vacations. Perhaps more vacation worlds could be introduced in Seasons? Either way, we need more vacation spots. A big forest is only so fun for so long.
As for ideas I'm certainly open to, usually anything, but these strikes me as interesting in particular:
- Farming. I like my Sims to be busy. So having to take care of a barn, fields with crop, and farm animals is quite desirable.
- Superheros. This sounds completely interesting IF we could choose our own costumes ( even if we want it to be just normal clothes ) or not need costumes at all. I do like playing with powerful Sims.
- University. I actually didn't like University in TS3, so I'm hoping for one I do enjoy here.