I like madocs idea for fast travel, give it a nice little story behind it and make it require a rare magical gem or something to activate it. the story could be similar to the story in stargate, an ancient race that left behind portals on different worlds, in SG, an ancient magical culture that is long gone. they were highly advanced, possessed great magic and knowledge about it, and one of the rare artifacts they left behind was weird stone plates hidden around the world, with inscription no one could translate, so no one knows what those stone plates are.
My idea for this is that the stoneplates got a hole in the middle where the gem would fit perfectly inside, and the inscription goes around the hole in circles. the gem is rare and very hard to get and is working as a energy source, and would after a while it will be drained and needs to be refilled, this way the player cant fast travel unlimited.
The way you travel between the portals are by dialing the stonetablet you want to travel to, each tablet has it own "number" and you can only travel to an already discovered tablet. The tablet you are traveling to dont need a gem to receive, when stepping through a portal you can choose to leave the gem behind, leaving the portal open but draining the power of the gem, also leaves it at a risk for being taken by thieves or other random bypasser or the gem goes empty and you will have to walk all the way back to get the gem again. or take the gem with you and instantly closes the portal.
Also like the idea of paid transport, but then the price would have to vary based on the length and how dangerous the trip will be, this would also stop the player from abusing fast travel. and for mounts, i dont see it to fit very well with sg, as mentioned before its hard to make it look good and work well with the physics in the game, if they are able to make it well i would love to see it, but rather not have it if its poorly made.