Legion should have been the end of a book, tbh.
(and also should have been 2-3 expansions all by itself. I always envisioned a planet hopping campaign to get closer and closer to the legion homeworld before we attacked them there directly. So Legion would be 'repel the invaders', 8.0 visit a bunch of planets, discover the 'legion destroyed all life' canard to be a lie, and gain many new allies to defeat them, 9.0= argus. Actually putting thought into zone design could have made for variety that wasn't all green, too.)
Then we come back to azeroth, time has passed, azshara has invaded the coastlines, the old gods return, there's 10.0 and then 11.0 right there. That would also help bring us back down to earth, so to speak, after all the space travel and demon fighting.