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Would it have been possible for a unified Celtic state to have been (eventually) established outside of Gaul during the time of the Roman Republic/Empire? Perhaps with a POD of Burebista being defeated by the Boii and the Taurisci and the destruction of the Dacians? Or somewhere in Pannonia or Dalmatia?
Edit: Then perhaps after the defeat of Burebista, the Celts siding with Caesar during the Great Roman Civil War