TOR tried this (before launch), it didn't really work out.
If Blizz can make it happen, I would try it.
I can see positioning playing a much bigger role with this than any of the other tanks, so this could be something new and fun (if done right).
Obviously, since the ranged tank has to stay out of melee range, there has to be some "proxy" in actual range of the boss (like the "shield" someone mentioned). In that case, I fail to see how it is different from a hunter or a loc tanking with their pet.