The mounts need to have some more in-game uses other than standing atop trees, on vertical slopes, killing dragons for you, or constantly out-racing your companions for teh lulz. When you splash out the 1K septims for the horse there should be a little bonus for doing so. So, saddlebags: extra inventory space which the player can access from a menu when clicking on the mount. Capactiy shouldn't be anything so excessive that'd unbalance the gameplay - say keep it the same as what the player can carry normally themselves. Just somewhere to store the resistance set of daedric armour, that poison snuff box you've never quite got around to using on the local Jarl, or the wabberjack. If you actually go the extra mile and add a bag model to the current horse use Jack Palance's face as the texture (a saddlebag with eyes).
