Definitely this one. The next conflict should be driven by an openly revanchist Alliance that wants to see its glory days restored and the Horde punished severely for its crimes. No holds barred and no punches pulled, with its own Theramore to ensure the playerbase knows the stakes (Silvermoon would be a good candidate, given its tie-in with Alleria and threat it poses as a gateway for the Void).
Alliance players would get to experience a faction war unchained from the likes of Anduin, and Horde players would (finally, after several decades) be on the side fighting for freedom and peace. Everyone wins.