I generally consider any 3rd faction concept dead on arrival.
It's not a matter of adding a couple more playable races then normal, we'd be talking about fundamentally rewriting the game from the ground up. So many parts of the game were implemented with the assumption that only the Horde and Alliance will be playable, to the point that we'd have something bigger then all previous expansions combined
before we even begin to consider any new endgame content.
That aside, I'm thinking expansion 5 is going to be based on the Burning Legion. We have a lot of indications pointing towards it. The bulk of the expansion's content will likely deal with us invading a world occupied by the Burning Legion. Argus is most likely, but Xoroth wouldn't be out of the picture completely, it really depends on whether or not Blizzard is ready to make a Draenei expansion or not.
An avatar like Sargeras used to battle Aegwynn, sure. But not Sargeras himself. I don't think Sargeras will (or should) be battled at his full capacity in WoW's lifetime.
I don't think there would be much of a problem with fighting Sargeras within the scope of WoW. Fighting him at full power might be tricky, but he could always be not at full power, or we could have some sort of unexpected advantage over him.