I really like the idea of it using CON, but others have pointed out that CON is so universally useful (and it gives a lot of health compared to many games) that polymorph would be too good to invest in as almost any class (unless it only really gives you melee bonuses or something.) Balanced right, I don't think it using CON would be that big of a deal, but it seems like it'd have to be weaker than your standard skill ability tree if it uses CON. Or perhaps you have to sacrifice something to polymorph (e.g., health? Willpower because it takes concentration?) Dunno. It's hard to make suggestions when we don't even know what kind of skills it will really add.
