You start loosely affiliated by story with Rome, but not part of any faction. Most of the unique content is for Rome, but you are not forced to join it if you don't want to. It's just that your experience will be closer to how vanilla M&B plays and you won't see everything the mod has to offer. The only other faction I know with some unique content is the new kingdom you can start in Judea.

that cleared things up, thanks for the answer