Modding independent difficulty?

[HoneyBadger]

Bear tribe could definitely be benefited by berserker units.

You could always give Wolf tribes actual wolves, as well. Not to mention the possibility of werewolves, and even their own type of berserker, the Ulfheonar (wolf coated, mentioned in various Viking sagas).

The only big challenge with that, ofcourse, is the inevitability of the players getting their hands on those special units, and the balancing issues that would then arise.