This might seem like an odd request. But here goes.
Sometimes I like to take things a bit slower and just explore quietly, especially in a new zone... but walking is too slow. If you're playing on mouse and keyboard, there is no way to control your mount speed other than toggling "walk"... And with all the upgrades (and the 30% skill point), your mount just flies through the world at break-neck speed.
It would be nice to have a "slow" mount which has some kind of anti-buff which ignores all speed upgrades, i.e., ignores your 30% buff and also ignores your upgrades that you buy at the stables. This would allow me to slow things down a bit as I'm exploring a new DLC area, for example.
I mean yes, it's easy enough to just keep "walk" toggled on, but it would be nice to have an option somewhere in the middle... so I could trot quickly to my destinations without blasting through everything at light speed.