Green flames has to be strongly tied to the lore if it's implemented. As stated before green flames are usually fel magic and that's more of a Burning Legion thing. As a complete Alliance fanboy I'd really not want to have anything to do with the Burning Legion unless it involves killing them.
Then again if it's a end-game kinda thing where you make your spells more powerful by adapting a little bit of fel magic, then yeah I'm in for that. Warlocks are after all former mages who dumped the arcane arts for shadow magic in order to gain more power.
Really looking forward to how they're going to play this one out.