Toddlers. Something with toddlers, dear plumbob. And a few more activities for the kids, too, now that they actually have time to do things other than work towards school/childhood aspirations.
Alternately, something like ambitions which allows for self-employment options and more work-from-home options.
I always get really excited over the idea of a Pets EP (and even more so now that I actually own a pet for the first time in 8 years and I want my kitty in my game), but once they're there, the pets kind of get neglected, or I get bored of them quickly. Pets was the worst EP for TS2 only because all of the other EPs offered so much more to the gameplay. The TS3 edition did better by adding horses, I'll admit, but the cats and dogs just left a lot to be desired.