Somairle

Cartimandua would make an excellent non-combat hero, and unique. As Queen of the Brigantes, she was allied to Rome. Her bonuses could actually be tied to being allied to non-Celtic factions. If possible, say, something like being able to recruit a foreign unit/units, depending on the ally/allies (maybe she can have a unique structure that does it? No clue), representing things like when Rome sent a cohort to defend her from her former husband, or just some kind of broader generic bonus, but something the Britons otherwise wouldn't have access to on their own.