Improved hair / skin / eye texture mods.
After that, fixes for the little annoyances & problems. (Like, in Oblivion, every install includes little mods like Shorter Grass, no enchantment saran-wrap-glow, Oblivion gate map markers go away when you close the gate, DaRN UI.... stuff like that.)
Of course, all of that (plus stuff like replacement bodies/etc) will all depend on how the vanilla game is. Gotta see what it's like before you can tell if you'd prefer anything changed.