Ok as a general Idea, i think they should overall be 'type-set', the types being Element and Jutsu Art(Gen, Tai, Nin, Fuinn, etc). Once in a host(if ever sealed), their abilities should be something that gives them a signature jutsu of the element type, and a fighting technique of the Jutsu Art. I also think that they shouldn't just have animals forms when not sealed. I think it would be cool if they had the ability to shift to a more humanoid form, something that shows they're smart and not just savage. Allow them to fight in a ninja form against others. Even if that host doesn't have that element, i think they should get that signature jutsu only and to be used only when accessing the bijuu's power. No custom jutsu to be made of that element revolving around that bijuu, that way we keep things from getting OP.
Raven (Wind + Genjutsu Type)
Wolf (Tai + Water or Ice)
Bat (Gen + Sound)
Gorilla (Earth + Tai)
Crimson Falcon (Fire + Nin)
Panther (Gen + Storm)
Shark (Nin + Water)
Lion (Tai + Lava)
I can't really think of any other good animals at the moment, but this is just an idea anyway.