I don't think blizzard will want to dive us right into another world ending enemy expansion right after legion, with the zandalar being such a threat in Mists I think we're going to use a south seas expansion as a skipping stone to the inevitable maelstrom/old gods expansion, I have nothing to base this on, just a feeling that even blizzard as story tellers know jumping from 1 world ending even to the next would be a terrible idea. They defiantly don't have the writers they use to they're basically crap now, but this is story telling 101 with that being said too, It would be awkward to deal with Zul after old gods, considering they were a minor enemy in Mists no way you can make him seem bigger and badder after an old god expansion.