I agree that the Kvaldir are likely to make a return, hopefully it will be the next expansion. It makes sense with the name Mists of Pandaria as well, since the Kvaldir travel in the mist and bring it wherever they go. They also kept saying something along the lines of "the mist is coming..."

It is possible that the Pandaren need saving from the Kvaldir. Maybe some wounded pandas wash up on the shores of Org and SW and ask for help. They lead us to Pandaria, where we beat down the Kvaldir. I doubt they will become a playable race, but it is possible they come with a new class to play.

As for Kul Tiras, I think it will be added at the end of Cataclysm, like a special event (think Gnomes and Trolls near the end of Wrath). That event could also include Undermines.

If the Kvaldir are in the next expansion, then they will probably throw the Naga in, too, since they are also fighting the Kvaldir. I'm not sure how that would work out with us, since we are fighting both. Maybe we join up with one temporarily to defeat the other.

I think fighting to save a race or two would be a welcome change from fighting to save the world.