Many times your horse will run off when it engages in combat, and you'll have a hard time finding it. I think it's a good idea for a mod to implement a horse whistle or something to call your horse back if it's near by.

a great oblivion mod was "call steed" I think it was a magic ring but either way, something that calls your horse to you immediately would be good. it would also be great to have more passive, less suicidal horses when you're fighting something big or a lot of enemies. Horses need an overhaul in general. mounted combat, saddle packs to carry loot, etc.

Oh, saddle packs would be a really useful feature.

I'd like this magic whistle for my follower, too. Damn thing is always disappearing in the scenery, unable to jump 10 cm in order to follow me.