I don't want a rehash of previous EPs. I know a lot of people want pets or seasons, but I'd rather see something totally new or at least not just a Sims 3 version of a previous EP.
What about some kind of festival pack that allows you to build fairs, parades, carnivals and other community events? There could even be different themes: medieval, steam-punk, sci-fi, career fair, kids carnival, gaming convention, sports events, etc. It seems like this idea would really draw on the community-based approach of the Sims 3.