Emerald dream won't be the last expansion lol, sargeras will be i feel like that has been an undisputed fact since forever. I can't see emerald dream being a main expansion in the least, cities wouldn't exist, and regardless about OMG theres so much lore about it, not really. There's the nightmare yes, but short of us facing just a bunch of minions of the nightmare it doesn't seem very likely. Yes it is possibly the size of the current azeroth in it's entirety but there's not nearly enough content to last for that long short of just making stuff up to be filler patches till we kill the nightmare which probably has something to do with an old god.