There are two things I'd really like to see:
.The Dark Below. I wish it was real, because Queen Azshara should take come back sooner rather than later because they have Neptulon to torture. She'd be a great female villain, and if N'zoth was included, I'd like to see it dying, and she ends up finishing it off, usurping them and gaining complete control of the sea as a formidable weapon. The expansion's last patch should also end with her opening a few portals for the Legion to invade through, including one under the sea for some sub-aquatic demonic races to come through.
--Also, having N'zoth could involve the Emerald Nightmare, and maybe accessing the Emerald Dream to stop it from spreading further. It sounds like a very resource-heavy expansion, but I'd like to see the water stuff more. If it didn't lead into a Legion expansion, then I'd let it go towards a What's Under Tirisfal?-Lordaeron of the past-Sylvanas-Emerald Dream-type expansion instead.
.An expansion that focuses more on the sci-fi aspects of the game, and we go to a new planet. The Draenei receive a distress signal from a planet that is being slowly invaded by the Legion, and we all hop aboard the Exodar to help them push the demons back. Alleria and Turalyon appear on the planet. We also make a stop off at a somewhat-changed Outland, Sabellian meets Wrathion, and SMV has healed a bit.