Level 20 Pathfinder!
I could consider running a solo or small group game at this power level, and beyond 20th level even. PF has tons of things to threaten high-level adventurers, and there's plenty of 3.5 materials to draw from, too -- not to mention 3rd party. There are enough good-aligned enemies to threated an evil PC, too. :P
Any consideration on setting or themes? I have some of the books for Golarion, the default PF setting, but not the adventure paths. We could do a homebrew setting, or work out something else. Do you prefer light hearted themes, or horror/dark fantasy, or anything else in particular? Preference on normal, Mature, or Adult game?