Yeah, 5th level if banned from casting and ranged attacks(we cannot count of all shennanighans players could pull off) seems like good level.Then let's give most of the casting the heave-ho.
It's so iconic for shapeshifters I object to sealing them behind a subclass.
Way too high says I. D&D needs to move on from Fear of Flying and it's not going to do that when flying keeps getting shoved in the high levels.
Only problem is that it might invalidate certain terrain challenges too soon. But then again, other classes have features that can do that too.
Then move treant form to 9th level and add elemental form at 13th level.