As an Alliance player, specifically someone with a human main, I'd love to see the fall of Undercity. However, trading Theramore for Undercity isn't quite an equivalent exchange. A better reprisal may be an orcish settlement in Kalimdor. The night elves have been losing territory to the Horde nonstop since the orcs' arrival on Kalimdor and it's about time they start to push back. I'd love to see the kaldorei, under the command of Shandris Feathermoon, push the Horde out of Feralas or Ashenvale.
Alliance players, what would you see as a valid reprisal to the loss of Theramore? Horde players, what would hurt to lose? Remember, in this scenario, it will happen. You can't claim the Horde wouldn't lose anything, that's cheating!