@Meddler/Phreak/etc. - How would one go about making/balancing an AP fighter? (plus concept!)

Diana and Elise presently are the only two true AP fighters in the game (though you could possibly count Gragas). Unfortunately, Riot keeps them intentionally nerfed because of their tanky assassin qualities, which makes them oppressive if they are ever strong.

In my eyes, AP fighters should be a sort of polar opposite to AD casters. They should specialize in DPS, not burst, while also being tanky enough to live long enough for extended fights. Nashor's Tooth, in particular, would be core on AP fighters.

But I'm not sure if this is what Riot thinks should be the case. Is it?

Oh, and if you have a moment, check out this Champion Concept. http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/aGuGtOEQ-celes-the-hand-of-justice