A few thoughts:
I'd add about six dozen voice actors to Oblivion so that every race had at least three or four unique VAs for their generic NPCs and any even remotely plot-centric character (Baurus, Jauffre, Hannibal Traven, Armand Christophe, etc.) had their own VA.
The Sims 3: Bring back the neighborhood-building blank slate from Sims 2. It is sorely missed in the new game (although, to be fair, you can bulldoze all the lots in Sims 3 for essentially the same effect, it just takes a lot of time and effort to build up a living, breathing neighborhood from scratch like that).
Medieval 2: Total War: The AI never got fixed to anything even approximating an acceptable standard. Even Rome had decent AI post 1.2, and Empire's reached the "acceptable" threshold with 1.3. M2 never got there.