I think a nice addition to the same old ho-hum reputation rewards (potions, weapons, jewelry, gear, scrolls, etc.) would be to sell war-steed cosmetics based on the faction mount, gated at the same level as the mount itself.
It would add a lot more customization to war-steed appearances (more incentive for players to buy your overpriced dyes *cough* -- I'd rather be able to buy the one color I want instead of the one plus three I won't use), and gives players a reason to go back and finish any reputation in earlier places that they didn't finish before. Besides, a lot of the reputation mounts are awesome. Many of them have very unique gear and tack that I'd love to use on my war-steed. They'd certainly look far better than the incessant tablecloths that keep getting thrown at us for festivals. (Those look really tacky. Please be more creative. Thank you.)
I think the only stipulation I'd add to this is to -not- require mithril coins to obtain it in any way. Faction currency only so it remains a real faction reward.
(And if this is a repeated suggestion, I apologize. I couldn't find it mentioned anywhere else.)