One of my main concerns is that one tree would become a tanking tree. This seems like a wrong move. It would inevitably be the beast master tree, and I don't think that is a perfect match.
I would more envision it as another hunter aspect that works along the line of other class stances -- and with a redesigned Tenacity tree. e.g.-
Aspect of the Beast (redesigned) is a hunter aspect that can be used with a tenacity pet. When activated, it switches your bars, and has the character "ride" the pet. All tenacity pet abilities are still possible through direct control. In this aspect, the hunter is completely focused on controlling the pet and takes no direct actions. Because the hunter is not firing his weapon they are able to much more actively handle their pet; They help the pet cause more damage and threat with a better understanding of their opponent's weaknesses and they help the pet avoid a substantial amount of damage similar to druids.Some pet abilities that are currently from pet talents would need to become baseline pet abilities. Other abilities would need to be strengthened for proper tanking and boss tanking.