MoP / patch 5.x will not be natively supported by any private servers for a long time. The amount of work needed to support new patches, and especially new expansions, is unimaginable. Not to mention that Blizzard have made it harder than ever to emulate the game (there is not even a realmlist file anymore) which causes further time and effort needed to reverse-engineer it.
I'm sure it will happen eventually. Developer groups will figure it out and get a workable system for it, which servers then base their cores on. Patch 5.x only just came out, so unless there is some kind of miracle, Cataclysm is going to be the highest you'll get for the next couple of years.