Assuming Classic is a success, what is the future of Classic servers? The most probable option is that Blizzard releases The Burning Crusade for Classic, following the same path as Classic.
But playing with Classic Beta has lead me to believe that TBC made several big mistakes, which weakened the setup of Classic WoW. In some ways, an alternate TBC, with several changes and essentially new content, would be a better future. Of course, Classic would have to be spectacularly successful for Blizzard to green-light something like this. And even then, they may not, believing that fidelity to what was released is more important.
The biggest mistake, in my opinion, was having the Blood Elves join the Horde.
In Classic, the Horde has a very strong identity. They are the monsters, banding together for survival. In the immortal words of Zangief from Wreck-It Ralph, "You are bad guy, but this does not mean you are bad guy."
The blood elves really weaken this. They're pretty elves. Blizzard tried their best to give them a dark backstory, but when you join a group where 4 of 5 characters are blood elves, it just doesn't feel like the Horde, not the way a Classic Horde group does. Now, maybe Blizzard did need a pretty race to balance the factions numerically, but that balance came at the cost of the Horde's identity.
What I would suggest is that the Alliance gets the Blood Elves, and the Horde gets the Worgen. Werewolves are classic monsters, and would fit in with the Horde. Of course, this would invalidate pretty much all of current WoW's story lines.
A later expansion could give the Horde goblins, and the Alliance draenei. Keeping the Horde's identity as the "monstrous" faction, and keeping the Alliance as the "normal RPG" faction would serve make the factions more distinct.