Aside from the suggestions, I say that you should go for Vlad if you're looking for an AP mid laner. Though watch out. Vlad's early game isn't that strong, he really comes into his own from lvl 5+ and he needs quite a bit of farming done on him, which can be harder at first since auto-attack dmg is rather low and makes last hitting difficult and his only viable farming skill at low lvl, his Q has a pretty high CD even with plenty of CDR runes/masteries.
I don't really see Yorick as a good mid laner, neither now, nor from before. Btw, I've played around with him a bit in a couple of games and the whole AP ratio on his W doesn't ruin him or anything. Sure, he has a lower dmg output, but he's still pretty strong. (almost won a 1v1 vs. a farmed Trynd in one game, got him to use his ult and then forced him to run away because of my own ult. Unfortunately for him, he ran in the wrong direction and got his ass handed to him by another team mate XD)
Further suggestions. If you're good, you might wanna try Anivia. She can hit pretty hard, has a lot of CC and thus she's good for any team and at early levels she's pretty difficult to kill if you're careful, potentially even more so than Vlad.