Blizzard has a LOT of characters that they aren't using, and who have a lot of potential. Blizzard can't even manage to work our own faction leaders into the story very well, barely giving Sylvanus and Genn any attention after the short cinematic slap-flight. What are the other faction leaders doing, picking their nose? And you want to bring in Wrathion, some baby dragon from two expansions ago? Pfft....good luck.
Personally I'd rather see Yrel make a comeback, and she's not even from this universe. And they killed old Maraad(he was a badass with a cool back story!) Lor'themar and Baine might as well not exist. I don't even know who's in charge of the dwarves right now. The orcs and Trolls don't even HAVE a leader anymore while Thrall is on vacation.
Wrathion? Take a number and wait, because Blizzard is too busy pushing Anduin while Khadgar waves his hands in the background.