Seeing as we've had the Celtic Face and Unit DLCs lately, it stands to reason their might be a gameplay DLC targeting Celts. Maybe a more 'what-if' DLC where the Celtic nation rises again in Ireland, throwing off Christondom and returning to old ways with a start date of 867.
Actually I'd say it's the opposite - if they were doing a Celtic-themed DLC, they would have tried to release the face and unit packs with it. At the least, they would have released the face and unit packs afterwards (like the case with LoR and the Mediterranean portraits).