#4
by Dersu
While there might not specifically be other races talked about within Celtic mythology itself, the ancient Celts certainly would have had contact with a large number of races. They are in the historical record as being in several parts of Europe, and seem to have had contact with Egypt as well.
Mummies dating from around 1000 BCE, which are of Celtic/Gaelic/thereabout origin, have been found in China, so the idea that they would have had some sort of contact with nomadic tribes is also not outside the realm of reason.
With that in mind, I think future expansion packs involving races similar to Vikings, Gauls, Goths, Romans, Greeks, Egyptians and Mongols is entirely consistent with the possible history.
Considering the suspension of disbelief required for seeing a hovering sled, I wouldn't find it strange to have my druid running around in Basque.
If you're talking about being able to BE another race, such as a Visigoth or something, well the OTM could probably just change the name to "Mythic Heroes" and create new low level areas in those places.