TBC should have come after MoP and WoD should have been scrapped entirely. The Blood Elves and the Draenei can still be introduced in WoW's first expansion, WotLK. Here's how things would have made more sense:
1. Vanilla transitions organically into WotLK after Naxxramas' fall; the Lich King unleashes the second plague as retaliation and the Alliance and the Horde invade Northrend.
2. The Blood Elves don't get their redemption arc until much later and can remain the Horde's second token evil teammate for much longer instead of being High Elves with green eyes.
3. The characters of Illidan, Kael'thas and Vashj don't get thrown away because of WoW's earlier story weirdness.
4. The Horde - Alliance war would feel more meaningful with the Horde's hardliners coming to the forefront instead of all the pro-war top brass being Garrosh's outsider alien orcs from another world.
5. Garrosh is introduced much later and his story arc could be about living up to the Hellscream bloodline and fighting DEMONS instead of... just about everything else.
6. The Legion doesn't launch another half-assed invasion of Azeroth right after their defeat in WC3. Instead, they recoup for years before trying again with their full might.
7. New TBC leads directly into Legion, no need for WoD.
8. WoW's overarching story elements like the Naaru and the Void Lords can be introduced much later and fleshed out more in new TBC.
9. Foreshadowing is cool - M'uru turning dark would be fresh in people's memories as they interact with Xe'ra and L'ura.
10. But most importantly, Illidan doesn't need a half-assed redemption arc because by the time new TBC launched, Blizzard would know that they want him to be a hero instead of a villain.