I would like to see Chocobo's first, and hopefully soon.
The ideas of previous posters about customizing them are really great! I would love to see that, see how creative people get with it.

Also, rental and have your own.
Some people aren't into the whole raising a Chocobo thing, so that's why I think we would need both.
The quest to be able to rent should be different then the one to raise or own (or whatever..battle bird... mail bird... etc.)
For owners, I'd like an ability where we can summon them while we're out and about. *Not an Item.* The ability could be the reward for finishing the quest and have a reasonable cool-down timer.

The Chocobo's should be able to travel all the same "field" places we can go on foot. I would not want to see anybody riding anything inside of any town.

I don't have a problem with using Garlean Empire... vehicles? Whatever they might be... BUT keep those within certain areas. I wouldn't want to see something like that right outside of town.

What would really ruin it for me is... Say I am strolling the paths outside of Gridania looking for harvesting spots and this swarm of players tramples past me, each on a different mount. It's an extreme and perhaps unlikely scenario, but I feel like that would start to look tacky fast.

Chocobo Carts! There is a guy with a cart right outside of Gridania. So maybe he becomes usable and you can pay him to take you to Quarrymill or all the way to Ul'dah. Though maybe this ought to be instanced? That way you don't have to wait for him to come back if somebody hired him to go somewhere far. lol

Airships... they tease me, I see them flying by when I look up... I see what I'm sure are docking platforms... >_> What's the deal guys? lol ^^

And to toss in something "random": CANOE!!!!!!!!
I forget which FF game had a canoe, but I remember rowing around in that thing! Maybe they'd be a Limsa-only thing but I think it'd be fun to go around fishing or exploring in one with a friend!