Concur with Zaarin ; I doubt we're getting the Celt-blob back. We are probably getting other ancient European non-Classical civs (the "barbarians") as the Greeks and Romans would put it), in the long run, and I fully expec tmore than one Celtic civ, multiple germanic civs, and so forth.
Ultimately, the idea that "they won't waste antique slots on Europe" seems to me a poor understanding of the devs' approach. They want to keep the number of choices unlocked automatically (ie, civ unlocks) limited to avoid overwhelming the player (they have acutally told us so) ; so they cannot just plop the celts leading to most of Western Europe - if they want a lot of Exploration European civilizations, they will need a solid amount of Antique European civilizations leading that way. In reverse, having a half dozen antique mesopotamiam civ, but they all become Arabia in Exploration, you end up with a lot of civilization that feel much more similar to one another than they otherwise could have.
I would more than expect the regional breakdown to stay relatively similar across the eras.