First things first. I agree when you say the class should be about enhancing and controlling.
I'm not really sure that I would focus on controlling enemies since utility for a melee class wouldn't need to disrupt the enemies much, the man-at-arms skill tree does that well enough. And also that would make it super duper op ESPECIALLY if this skill tree is going to scale with CON as you(and I) think it should. Imagine a mage with your polymorph skills using it just against enemies. . . Also I'm not sure that strictly benefiting STR melee fighters is the way to go either. You kinda put rogues in tough spot since you kinda don't bother with DEX at all.
With that being said, I think the utility that a polymorph tree would need to provide a melee fighter would be better suited being something that protects him/her from environmental hazards or increase his/her damage output(i.e. decreasing ap costs/increasing crit. rate|damage).
I think something that could make them immune to clouds (poison/electric/smoke) would be a really neat thing to do. While I have no idea what the skill would be, they could be different skills for instance, you could have something that allows them to be able to see through smoke clouds (I'm thinking something simple like cat-eyes). Another idea is something that makes it possible to walk on ice(not exactly like the scoundrel skill that avoids terrain). These would give non melee classes reason to consider it.
As for the increasing of damage output, I like your claws idea actually. Kinda covers the damage output thing. I do think that there should be a couple of AoE attacks however, in the spirit of general utility for melee fighters.
The next problem I have with the skills(which I already sorta mentioned) is that they have 0 drawbacks. If the polymorph ends up being the way that you've made it, I can't imagine not having ALL of my characters sufficiently trained in the skill tree to have some of those skills. I mentioned in a different post that I would have some drawbacks (akin to the ranged/power stances). The easiest drawback to come up with is for the Blubber skill. Just lose x% movement speed.
You have good ideas, but I personally think that you went a little too op and I would focus more on the characters' interactions with themselves and hazardous environments as opposed to crowd control.
