Have you tried City of Heroes/City of Villains? It probably has the best character customization of any mmo, and I think CoV's Mastermind is the best class in any game ever. The Mission Architect system is a lot of fun too. Of course, the game isn't without its flaws. There are only a few different mission types so the game can become repetitive fairly quickly.

I'd also recommend checking out Dungeons and Dragons Online. The game mechanics are completely different from pretty much every other game and the ability to multiclass adds a lot of variety in character options. As an added bonus, it recently went free to play.