Frankly I feel like that whole 'undefeatable god' that you will then spend the expansion farming trope is a bit tired, we've had Arthas, we've had the 'end of the world' scenario done and covered with cataclysm, if anything that whole 'join together alliance and horde to defeat your greatest enemy yet! has never felt so overused. The Burning Legion arriving feels like an end that they can't top once they pop it.
Oh you defeated Sargaras? Our next expansion is a sky expansion, where you fight sky pirates. See? It doesn't make sense on a progressive level. If anything I just don't think they should play the Burning Legion card yet, if ever.
Now... The South Seas! The Seas around the Maelstrom! Island-hopping! and exploration! They could add goddamn guild owned big ass boats which you can upgrade and decorate and colonise islands for your guild and hey, where's Azshara yknow? You've got Undermine and loads of big fucking islands out there already in the lore, make a explorative expansion, add new dimensions to the gameplay, aint nothing like guild owned boats that will bring back guild identity within servers. Keep your fantasy tied down a bit and the big moments will actually feel big instead of 'oh look another demigod, lets farm'
South Seas Maelstrom expansion is the better option, fact, and no this isn't subjective, for the reasons outlined its a better option.