The Summon Familiar spell is boring.  It would be fun to have a random creature be your familiar, perhaps being influenced by your schools of magic or profession.  Different creatures could have a different bonus, whether specific to the unit or global to all of the units in the stack:

Familiars can cast spells and then have additional abilities:

Ogre Magi:  A juggernaut with spell casting heh heh

Wisp as currently available but with spell casting as well

Hell hound: Candlecloaked, Can teleport and use fire breath once every 4 turns

Treant:  Can cast root once every 3 rounds, (see mqpiffle's ROTM mod) and negates swarm bonuses to enemies attacking it 

Shrill Familiars:

Air shrill:  + 10 dodge to army's ranged defense and + 1 air power to Sov's air spells (must be in same stack)

Water:  Enemy fire damage reduced by 33% for all allies and + 1 water power to Sov's water spells

Fire:  All units receive 33% protection from cold and +1 fire power...

Earth: All units receive 33% protection from lightning attacks and + 1 earth power for Sov

Fantastic Mounts:   (coupled initiative so sov and mount always act as a pair)

Unicorn:  Teleports once every 3 turns and offers +15 spell resistance to the whole stack

Wyvern:  Flight and attacks with a poisonous sting

Pegasus: Flight and reduces sov casting costs by 25%

War Bear: Armored brute with maul

Ice Drake: Flight and ice breath every 4 rounds that damages and slows any units caught in the blast (3x3 area)

Basilisk: Six legged beast that can turn enemy units to stone once every 4 rounds and has a poisonous bite