Although the first historical reference to the Picts is later than EB, surely a similiar people dwelt there before the romans showed up? I believe an inclusion of another faction in Britain would be excellent, be it Celts or the Brigantes you mentioned. It would be great to play an early "Scottish" people (i mean simply that they live in what is now Scotland) , and it would add a more difficult element to the Casse campaign, who have quite an easy time of it against provinces of non- ambitious Eleutheroi.