Its "Europa" Barbarorum then my guess is that another N-W European or Spanish faction will be there and not something from the east (no offence). My guess is a Belgian or another Germanic nation is coming up next in the previews.
But to be honest I am happy with every faction the EB have in mind and published so far. They are all great and exciting to me. Would be fun something related to the Batavii being published/mentioned within a Germanic tribe.