There'd be plenty of factions to choose from, if they went the extra mile to add a totally new 4th and even 5th faction(s). Maybe in a paid DLC, but def not for launch.
The inca / mayans
Chinese dynasties? (might be visually similar to the samurai, but they definitely had way different weaponry)
Greeks
The mongols
Any of the middle-eastern empires with their turbans, black mascaras and curved swords
Roman Legion
etc.