The question to start with, would be what purpose the individual shapes have.
Boosting a specific attribute, trading whatever attribute Polymorph revolves around temporarily for a different one?
-> Could work well with Constitution. Scale with Constitution, take a penalty on Constitution upon use, achieve a boost onto a different attribute. Could even be stacked, but the loss of Constitution would mean (natural) diminishing returns while also turning the character less durable.
E.g. "-20% Constitution. 25% of Const is added to Strength"
Granting specials skill only available to a specific form, bending the rules?
-> Possible, but it should NOT use the primary attribute of the Polymorph tree, especially not when it is constitution.
It could be interesting to push the Polymorph combat wise in a similar direction as the Geomancer, constantly balancing between glass cannon and tank, except that the bottom line for the Polymorph is "tank", while the glass cannon configuration develops with skill use. Naturally low on the attribute(s) usually required for the secondary/primary class, with the option to boost in atypical directions if required, and a small number of active skills.
