Wow lots of good suggestions in here! If the sims was a little more like life minus a few things it would be perfect.
Dance animations that don't look completely idiotic would be great.
Actual support of the custom content community is an absolute must.
Of course the most important thing that needs to change with the franchise is getting rid of EA. The game would probably be better taken care of (quality and patching) and a little more forethought could be put into expansions.