They should have just switched owner for the foreseeable future. (permanently) Undercity could be renewed as it was before the Arthas destroyed it. Honestly that is the ONLY thing that I really want in WoW still. But I think I won't ever get that.
Tbh I when I bought the game back in Vanilla. I heard about PVP and taking cities etc. Only to find out that yeah you can take them (by defeating the lore person in the throne rooms) but not really TAKE THEM. Obviously considering that this is not really feasible (atleast not back then), I was not that disappointed. But it would have been cool