I've been playing a ton of Xul on Heroes of the Storm lately. He's an absurd amount of fun, between his powerful and unusual Scythe attack and his fantastic control and a trait that's both powerful and flavorful, it's just awesome.
The obvious place for him would be in Diablo, which is of course kind of backwards, given that Xul is officially the Necromancer from Diablo II.
Xul's role in Diablo is more or less filled by the Witch Doctor. Both raise the dead to fight for them, but while the Necromancer has a more gothic theme, the Witch Doctor has a shamanistic one. The Necromancer is explicitly Lawful Neutral, whereas the Witch Doctor is more of a good alignment - though arguments could be made whether he's Lawful or Chaotic.
One of the things that's interesting about Xul is the way that he effortlessly raises skeletons from defeated minions nearby. This actually makes him feel a bit more like a Warcraft III Necromancer, who had an ability that could be toggled on to automatically raise any nearby corpses to create skeletons. In Diablo, you could imagine making a trait where super-flimsy skeletons are created any time you kill an enemy (or perhaps beef them up a bit and make it a chance.) This is relatively close to the Witch Doctor's Fiendish Sycophant trait, which instead creates Fetish Warriors.
Spectral Scythe is really the most unusual ability Xul has. To reiterate, you cast this at a targeted location. It appears there a second later and then travels toward you. It's kind of a skill-shot, except the skill comes in where you position yourself, rather than where you aim the shot. You can actually curve this by moving after casting it, and you can wind up getting a long range if you cast it and then move in the opposite direction.
Such an ability could work in Diablo, but I think it would be an awesome ability for Death Knights in World of Warcraft as well.
Anyway, I just wanted to gush about the new character who's probably going to supplant Leoric as my favorite in the game.