Renekton mid?

He was always fine in mid. Any champion that has good diving capabilities can do well in mid against mages as they don't usually have escapes. Xin, Wukong, and Renekton are good examples as they just jump on champions and the other person usually can't get away. The problem is that later in the game, you will run into problems where you will possibly need a tradition mids power, like their burst or assassination potential, to win you the teamfight and you're stuck with a tanky bruiser there. If you can have someone else fill out that role, you are fine, but if not, you will be in for tough times if you didn't snowball hard in midlane and push your advantage early enough to win.

instant death to any squishy mage..

Would be an interesting counter to some mages, but others would probably wreck him. Generally ones that can keep a very safe distance like

Any champion good against mages can mid. The only problem with Renekton mid is that his team would need someone to replace the role of APC or Assassin.