Visually these factions have great themes, you're definitely very creative. I like how they're kind of based on biomes too. Some theme ideas off the top of my head:
-"Spirit Horde" Crystalline / Portals / Totems / Spirits / Ethereal (could go with either shamanistic or crystals / aether theme)
-"Horde of the Volcano" / Lava / Golems / Dragons / Kobolds / Minerals
-"Horde of the Desert" / Tents / Yurts / Kodos / Nomadic / Harpies / Wind or Sky magic
You've probably already thought of these and there are many more, just based on the tilesets even. Cool project, I wish you luck!