Imagine how cool it would be if the night elves took over the ruins of the Undercity after the battle. They could drain the sludge and fill the canals with glittering moonwell water, open up the rooftop to the stars, and return life to Tirisfal Glades. It would give Blizzard a reason to update the textures and smooth out the blocky 2004 graphics of the city, while giving it a fresh new face (but of course, keeping it familiar to long time players).
There is a lot of potential to remake some zones with all this movement of races. I mean the night elves could move to Silvermoon instead and remodel the city. The blood elves or undead could move into the ruins of Darnassus. It would be a very interesting project for the artists to mix the styles of two different races architecturally. It's an interesting concept if you ask me