As far races go I think some anthropomorphized animals could be a cool idea. By mirroring some races from native animals, of North America at least, some interesting dynamics could be put in place. The cultures of Arcadia could be as unique as they are because of their particular circumstances.
For instance, a migratory race of sentient bird-people could be responsible for trading material goods from distant areas that would never have interacted otherwise. By traveling their yearly flyway they trade goods amongst many different cultures.
A hooved, herbivorous race could be responsible for spreading crops like maize, beans and squash across the continent. Maybe they originated from a plains area and could resemble buffalo or pronghorns.
These cultures may look technologically primitive by Avistani standards, but their minimal use of technology is bolstered by their advanced magics.