I'd prefer INT + CON hybrid. (to symbolize knowledge and endurance to do so)
Btw...
I'm actually not really into this shifter stuff in general. But... if it's not only about transforming yourself into animals/whatnot, but use it in more diversive ways... like turning a fierce warrior into a fish in the middle of the desert... Or like DnD some partial transformations, like growing wings, or scales.
Or in conjunction with other skillsets, like summoning a wolf and turning it into a giant scorpion...
That I could find interesting...
