They have old PVE ones on like the Zul'Gurub mounts, the Core Hound, and the Plagued proto-drake, so why not add in a few of the old PVP mounts?
I'd say add 1 from each season, and a good way to choose which mount goes up is by a poll on their website, along with screenshots so people can decide which is the purdiest!
From the linked list (http://www.warcraftmounts.com/pvp.php), I'd personally go with Swift Nether Drake, Furious Frost wyrm, Cataclysmic Twilight Drake, Malevolent Cloud Serpent and the Primal Gronnling.
What are your thoughts? A good idea, or do you think it'd take away from the rarity of those mounts?