Just an idea, don't know how popular it will be, but just like you have RL mounts that might be better at different tasks, you could have mounts that specialize in things. Like one that is better for carrying stuff, one that is swift (chocobo preferably), one that would go longer without feeding it (assuming they introduced a feeding mini-game) etc, etc.

It would be cool to see mounts brought to a new level than has been traditional in mmos.

Something they did have in a previous mmo, was mounts that added to your stats, if they were to introduce mounted combat.