That's where the maybe comes in. At least with cross-faction instancing the possibility of going Dwarf is on the table. Without it, you'd see entire guilds going Alliance just to take advantage of it, essentially creating a reverse scenario of what led to our current Horde-dominated endgame hellscape.
This could work, for sure, but Blizzard would need to go all-out on such a feature and it'd likely be a lot of work. This wouldn't be something they'd add in a .x patch.