So I was thinking about mounted combat and a came up with a simple idea. Just add a few special mount abilities that the player would have access to while mounted.
Basically, the mount would function like a pseudo-pet. They wouldnt act autonomously, but instead, while the player was mounted, they would get access to special abilities/attacks. These could even be flavored based on the type of mount. For example, a wolf might get a powerful bite ability to attack melee opponents with and a leap ability that allows for an extra-long jump. A horse might get a rear-oriented kick and a charge ability.
Of course, it would have to be balanced in some way so that it wasn't always advantageous to fight while mounted, but it would be an interesting and simple way to allow mounted combat.