What about a future lore raid where there is an enemy so powerful mere players couldn't even scratch them. What if some how each class had 1 or 2 characters in lore that they would become while entering the raid. Say We had to kill an old god at full power. Shamans would become thrall, Mages would become Jaina, and priests would become Velen. Your spells and everything would stay the same, but health healing and damage would be increased by like 10 times. The raid bosses would also deal increased damage and have massive health pools.
I guess in the end you wouldn't need to scale anything, but scaling it all up is part of the fun of playing powerful lore characters. So if it happened in MoP. a dps would be doing 1.5m dps as Sylvanas the hunter etc, and a tank would have 10m hp. You would also look and appear to be the lore characters. It would be explained some how in the story why you are playing as those characters, so don't worry about lore problems.