Well, based on my experience in 3.5, that has the potential for a prestige class, if there isn't one like that already. I don't know about all the technical details, but you could put some interesting riding-based skills or feats in there. It definitely is an interesting idea for a campaign. However, I'd be reluctant to make an entire party out of bards because a bard is like a swiss army knife. While it can do its main job (cutting things that you would cut with a knife) exceptionally well, and it can do other things adequately (screwdriver, corkscrew, etc.) it doesn't do them as well as a tool designed for that job would do. While a bard -can- check for traps, open locks, cast spells, and fight monsters, there are other classes that do the same job, just better due to game design. So it's a neat idea, but I wouldn't want all my PC's playing as one.