You know, I was just thinking about this today. My Sims are usually slim and "muscular" because I have them do a lot of exercise, but they really never get as ripped as I think they'd get from all that exercise.
My male sims are not that slim in the game because I never let them do the cardio exercise. That might help on their slimness.
Not really a mod, but I am going to be really happy when I see some new eyes. The Sims 2 had awful eyes, and The Sims 3 has even worse eyes. I know some people have complained in the past about the freakish realism that "real" eyes bring to the game, but the vacant doll-eye stares of the base games bother the hell out of me.
It'd be nice to have a TS3 mod where occasionally a baby is left at the door, and your household has the option to either take the baby in or send them to the orphanage. That would be cool, esp if they allowed the supernaturals expansion to be applicable so then you'd end up with a vampire baby or something