Rackhir, I really did start out the idea to be able to let PCs play someone like Beorn. Like I said up there, I can play all the homebrew worlds I want, but I need to get more bodies in the seats (it's hard to find a lot of home-grown gamers in rural central Virginia, especially as a recent transplant).
You have all made good points. I want to keep it as close as I can to Tolkien, but also make it something that doesn't break the game. Humans are hard because I am having to balance the Numenoreans (Dunedain, same thing, I just learned) with the others. This gives "normal" short-lived Men a chance to stick it to the cocky guys from Westernesse.
I'm thinking that I will let PC characters take a "latent skin-changer" feat at first level (and only at first level) that gives some benefit, like an ability score increase (no half-orc PCs in my campaign makes for no PC races with a STR bonus) or something like scent or low-light vision.
Then later on, maybe when the PCs reach the character level equivalent to the normal animal's hit dice, they would be eligible for the full skin-changer benefits with another feat. This seems to fit with the idea that not all of the Beornings could change their shape, even if they were in the same bloodline.
In order to make it so that it isn't just great earlier on but not so neat at higher levels, I'm making it so that the BAB and HP in the characters animal form will be the same as in the human (or Man) form. I may even figure out a way to increase size over time (using advancement rules in the MM, maybe), to scale up the physical attacks as the characters progress.
Just thinking out loud right now, but I think with the suggestions I got here I may be able to work out something good.
Example latent skin-changer feats:
Wolverine skin-changers get one rage per day
Boar skin-changers get CON bonus
Eagle skin-changers get WIS bonus (?)