IF there is another class the most logical as has been stated is to have a mail wearing class. The best fitting would be to have each of its three specs compete for a different kind of mail, particularly to give competition to INT and INT/SPR mail which is exclusive to shamans. This way all gear would have a minimum of two specs rolling for it, with the exception of INT plate which is something they probably can't do away with unless they want to change Holy Pallys and remove all INT plate.
The reason I would lean towards there being another class, 'hero' or non is the way the raid tokens are currently set. Prior to Wrath each tier piece had three classes rolling for it. Wrath changed that and so one tier piece had 4 classes while the others had 3. With MOP two tier pieces have 4 classes rolling for them and there is one tier piece with 3 classes. Adding another class balances this because we get 4 classes rolling for each tier piece.
What I would hope for the next expansion is rather then leaving Azeroth and battling the burning legion on their turf, we defend Outland from a burning legion invasion that serves as a reminder to both the Horde and Alliance who the real enemies are. This way it gives Blizzard more time to do the expansions leading up to Sargeras where we sort out all the problems on Azeroth so we are actually sure that Azeroth won't get wrecked while we fight the burning legion.
The premise of another Outland expansion is not Cata wasn't to bad lets rehash more content, it's more lets survey Outland after we saved it see the land starting to heal (if thats possible) and then some burning legion burst forth into Outland and do some crazy stuff and spells that cause Outland to change, so we are not going through that waste that is Hellfire Peninsula but a new zone that has some of our memorable characters from questing at 60 as a link to the previous expansion not the whole basis for the current one.