I think it could work from a gameplay perspective (although it'd be nice to finish Hrothgar and Viera in terms of gender and head gear). I'm unsure about the lore though? It seems everything has been written to have some kind of duality so adding a third clan out of the blue might be weird (from that lore perspective only, mind you).

I understand that what WoW did with allied races kind of worked because there wasn't that "exclusive duality" to begin with.