[Gregstrom]

Man, Tir, Eriu, Fomoria and Marverni pretty much sort out the Celtic background/mythology for the British Isles between them (okay, Marverni is a little more Gaul than Gael I guess). The Anglo-Saxon bits are dealt with by the Skandic nations, more or less.

I'd definitely be interested in seeing Kalevala (about which I know nothing) and even the Eastern European/Russian mythologies in Dom3.