Speaking from my limited experience from working with Arthurian: Total War, there's certainly enough diversity of units and factions etc. to make a Celtic mod, but finding an appropriate time-period might prove a little problematic. In particular it could be difficult to find a time-period in which all the "Celtic" peoples were equally active (the British kingdoms have been much reduced by the time the Viking raids in Britain really kick off, for example) Still, I'm sure a way can be found.

I actually mentally sketched out a Celtic faction unit list for a European fantasy/mythology mod, based mostly on mods like ATW and EB, with some mythological touches, but I agree that a historical approach would probably have more to it. Good luck, anyway.

Antagonist