One other thing to consider: How does RMU handle the physical change? I mean, when you change form, do you take on all the stats of an average individual of that creature (including level), or do you stay your current level and just take on the new physical features (stats, claws, Armor Type, etc.)?
E.g. A level 1 Beorning or Shapechanger changes into a bear. A medium bear is level 8, a Big Bear is level 12. Do you get the stats of a level 8 bear, or a level 1 bear?
I think you could do it either way. If you take on the level of the bear, then you'd need to balance the class by having it turn into a weak bear first, then a medium bear, then a big bear.
If you keep your own level and just take on the physical characteristics of the bear, then that is inherently balanced, but you'd have to build up skill in things like unarmed attack, bite, etc.