Clearly not a lot of popular support around here for another faction in northwest Europe, but I personally agree with Brennus's sentiments. The Atrebates position seems interesting, though if it came down to picking one and only one additional faction for the region, I'd probably prefer to have the Brigantes.

The main draw to Atrebates (to me) would be the possibility of starting with territories on both sides of the English channel. However, it would seem that given the way the current campaign map is in EB1, their holdings on the north side would be synonymous with Cassemorg, which is Casse's only starting province. So unless the EB2 team decides to squeeze another province into Britain (which would mean cutting one from somewhere else, a very unlikely scenario), the Atrebates would realistically be starting with one of the two Belgic provinces. And then you're left with the question, "If we're going to include a Belgic tribe as playable, why the Atrebates when there are other good (and probably better) contenders to represent a Belgic faction?"

I think a Belgic faction would be great, and I strongly suspect that one will be included in EB2, but if we're debating a best choice for a new faction with a British presence, then I think Brigantes are your best choice. I think the main reason to include a "rival" for the Casse is that they're boring to play. I haven't been playing EB very long (maybe a year) and have only been visiting these forums a couple months, but I suspect most players never bother with the Casse, and those that due probably find themselves establishing a hegemony over the British Isles before looking toward Continental Europe, where the real action (and challenge) is. The Casse are not threatened by any other factions for a long time and, besides financial difficulties, there's nothing to prevent them from slowly (or quickly) conquering Great Britain and Ireland. Adding the Brigantes in their titular province would change things a great deal, though admittedly I don't see it making campaigns more interesting for players of other factions.

I know there are candidates for playable factions with much higher priority in terms of historical significance (and information), but if I understand correctly there will be room for at least nine additional factions in EB2, and the only three are confirmed so far, leaving at least six more slots. By the time the team gets down to selecting that final faction or two, I think the Brigantes look pretty good in comparison to the remaining choices.