I'm only level 21, but for the longest time I did the same thing. I focused on AP casters doing solo-mid. So far, I've really enjoyed playing Fizz, Vladimir, and Malzahar in that role, with Fizz more oriented to being an assassin who just happens to be solo-mid.
Recently I decided to give tanking a try, though, and since I have all AP runes, I gave Cho'Gath a try. I've been building him as a tank/mage hybrid. I go for an early philosopher's stone for hp/mp regen to keep me in lane forever, and then build into a rod of ages, force of nature, frozen heart, and rabadon's deathcap. Most games, however, are over before I ever get to the deathcap. This gives me awesome regen and great defenses, and makes rupture and feast hurt like a motherfucker.
I started building as a pure tank, and while I still do that in some games should it call for it, I find the hybrid so far best. These aren't ranked games or anything, but having that extra bit of lethality makes you a bigger target, which is just what the tank needs to do.
And occasionally you eat their face.