I don't know much about these things, but I always thought Familiars were supposed to be representations of their summoner.  The current familiar is really odd since it's this demon-looking thing even though it is for Kingdoms, which are the "good guys."  I think an interesting feature would be adding to Sovereign customization the ability to choose what your familiar looks like.  It doesn't have to be full blown customization that is Spore-like, but being able to choose a bear or a wolf/warg or whatever would be interesting.  I searched for similar posts, and apparently in old beta versions there was a dog that was summoned by a token, but I've never seen one of those, so maybe the art is still in there.  Also, to further reinforce it as a Sovereign representation, Familiars shouldn't level up on their own, but along with the Sovereign (this is a debateable point for balance's sake, but it's interesting).