I quite like the idea of Polymorph being based on your constitution, at least partially. I don't think it's "too good of an attribute," but it is useful to everyone to some degree.
It is one of the most important attributes in the game for everyone.
Making a skillset that improved via constitution means that you don't have to spread your attributes pretty much at all (Doubly so now that your AP aren't tied to attributes). You will get a bountiful overflow of HP that will turn you into an unstoppable monster AND you will be doing tons of damage and CC in return.
As well because constitution is absolutely important it means that Polymorph becomes THE crossclass to get regardless of your build and overshadows all others.
Unless Polymorph gives you sizable penalties to offset the huge constitution bonanza... It really REALLY shouldn't be used.
