I think mounts beyond Chocobos are a good idea because, as other posts have pointed out, colors can only go so far even with customization with armor, saddles, etc. Mounts beyond Chocobos need to be creatures familiar to the FF series though, in my opinion. Wyverns, Coeurl, Adamantoise, smaller scale Behemoth; creatures like those who have been in several FF games throughout the years.

This may be stretching it, but the summons that have followed along the game as well (Ifrit, Oden (the horse part, like an Ixion in X), Bahamut,Fenrir, etc) could possibly be mounts? In the last Letter from the Producers, they said something about an Ifrit battle - perhaps defeating him would grant him as a summon to be mounted? I'm not sure how well it sounds on paper and how much better it would translate in-game, but it's just me putting in my two cents.

I think you're doing a great job SE :3