Admittedly the title is a tad redundant, but regardless, how could this situation arise with a POD preferably after 100 BC at least? The Celts (as the Gauls called themselves) had a rather advanced civilization and could form large confederations or hegemonies when pressured, but even without threat of Roman aggression surely they'd eventually consolidate? So what I'm wondering is what Celtica without Roman subjugation would look like over time, whether politically, socially, culturally, etc. I do know that they had multiple forms of government in various tribes, as in whereas the Arverni had a sort of high-king figure claiming divine right the Aedui had a system more resembling a republic, at least in the eyes of the Romans.