You can't rebel to the rebel faction without problems, and to have a shadow faction one would have to be cut. Personally, I think there should be a shadow faction, such as the Averni for the Aedui, roman traitors for Rome, another british tribe for Casse, etc, but I can easily see this being a nightmare for the team, so I would say it's only worth considering when we get to EB2 for the Romans, maybe for seleukia but again, with the complexity of EB, having it vary per faction would be a enormous task.