Basic concept: Each of the non-base skill trees, should have an added passive skill added to them. Putting a skill point into this give you a custom mount based on that skill tree.
Examples:
Vampire: Instead of a mount, you travel as a swarm of bats
Werewolf: Turns you into a werewolf for fast travel
Psijic: Time bending. You essentially bend time around you, making it look like you're moving at mount speeds. Give it animations like the flash or other fast/blurred motions
Mages Guild: Your character opens a portal and steps in. You travel like a normal mount as a small dancing light and once you dismount, a portal opens and your character steps out like they teleported via portal.
Excavation: A wagon, with the back full of a chest, digging equipment, notebooks and scrolls, etc.
Overall, we all likely have a stockpile of skill points and this would be a fun way use some of those, as well as give something nice to unlock once you've maxed out the skill line. It would give more creativity and variety, as well as just being a fun way to add more mounts with a lot of work. What all do you guys think, and what types of mounts would you like to see added to what skill trees?