It would be cool to be able to summon magical creatures as mounts.  The sovereign could cast a spell to change a tile on the map into a herd of these other worldly creatures or transmute a herd of horses into the new herd.  It could be researched through the magic tree, and maybe limited to one 'extra planar' mount per faction.  We could have dread steeds, fire grizzlies, crystal taloned raptors of the ice plane.  The mounts could also change the attack type of the unit its respective element.

But i agree bear cavalry would be awesome.  Although the arms race can be pretty devastating.

I'm looking at alternate cavalry like a map resource, you have to put a building on it and it produces 1 unit per turn and military units cost x amount to build.

I could see bears being slower than horses but being tougher and giving a damage bonus, maybe a negative moral modifier to enemies.

Having three or four alternates to horses would be cool, you could have standard horse mounts whose strength could be a greater variety of weapons available while mounted due to the easy of riding them.  Then you have a slow tough mount(such as bears), maybe a really fast mount, a high damage, low durability mount, and a high maneuverability mount (i.e. flying).  They could all be balanced by cost and their different advantages/disadvantages.

If nothing else I am interested in modding this in myself; I'm having a flood of ideas about this.