Seriously, though, I like the halflings vs. gnolls idea mentioned earlier. One alternate possibility is to use thri-kreen. Perhaps make them a bit more insectile or hiveminded, having them boil out of huge 60-80 feet tall anthill like mounds in the plains of the halfling lands. They swarm over the halfling villages like a plague of locusts, hopping in, feeding on some there, and taking others back to the hive to be eaten alive. Or maybe go w/ the halflings vs. gnolls idea above and use the thri-kreen against the gnomes.
The dwarves could be fending off an invasion of destrachan and their grimlock slaves. Destrachans are Int 12, apparently - according to the SRD, they understand Common, but don't speak it. You could probably handwave some subsonic communication between themselves and the grimlocks. Perhaps a cabal of advanced Medusae as the shadowy powers behind the destrachan advancement...
Instead of Goliath vs. Yak Folk, perhaps a nearby volcano has opened a rift to the Elemental Plane of Fire, and a Salamander army has come through, bent on destruction? Or maybe use them against the Elves instead, burning down entire swaths of forest.
HTH!
