Honestly, as an active mitigation tool...I would lik to see it something like:

Inner Beast: 4 second duration, takes 3 of 5 Wrath stacks, mitigates 30pct of damage. Remove the healing aspect, and place on it an enmity generation. That would make it perfect imho.

It would be more readily available for use when you need it, but you still have to balence keeping up your wrath stacks and mitigation buff with everything else you can do...cool downs, overpowers etc, the things that do not generate wrath will give you less mitigation over time. I would probably also give Tomahawk a +1 wrath too, so a hawk and the first threat rotation would be enough to start up your inner beast.

But that's just me, I feel that right now beast has the ability to really make this calls a great active mitigation class, which is fun in other games, but it's just not there right now. It needs to mitigate more, but last less, and be able to be cast more often to make it perfect.