EK - Forsaken, Argent Crusade, Blood Elves, Humans, Dwarves, Gnomes, Twilight Hammer, Red, black, green dragons
Kalimdor - Orcs, Darkspear, Tauren, Night Elves, Tolvir, Draenei, Earthen Ring, Cenarion Circle, Aqir, bronze dragons
Northrend - Lich King & Scourge, Titan Keepers and their armies, Vykrul, Taunka, Nerubians, Ebon Blade, blue dragons
Pandaria - Mogu, Mantid Swarm, Jinyu, Hozen, Pandaren, Zandalari, Sha
It would seem to be the places with the titan production facilities would be the final two, so Pandaria vs Northrend. That would be an interesting fight. If the dragons instead alligned as the Wyrmrest, and were Northrend, that combined with whatever the Scourge could rez, but then again the Mantid has a swarm. fun!
All this does is make me want to see a RISK game including all four continents! Are there some already?