@Zantetsuken

Right idea, different implementation than I'd like. No impassable areas due to lack of a mount (rental or otherwise). You should be able to reach anywhere over time no matter what mount you ride...

However 3 fair(ish) options could be any of the following (In my opinion of course)

A: Certain mounts travel faster on certain terrain and all others travel at rental chocobo speed. Normal choco's get neither an increase or a decrease

B: Certain mounts have accress to shortcuts, ie: a gob could lift a branch out of the way in Grid to cut across the map slightly (no content allowed in these shortcut passes) or a winged mount could glide down a mountain side. No shortcuts from one end of a map to another, just something to shave a few more seconds off than it takes to switch rides.

C: Certain mounts move slower but are resistant to attacks or a chance to scare off level X mobs for a given radius improving your chances of getting to that hard to reach camp or whatever.