Nothing's set in stone
I can make Brittany recreatable, but then it'll be Brittany + Tourraine, Maine, Anjou, Avranches, Poitiers, Thouars -- 12 provinces total.
Big advantage: Brittany is unclaimable without conquest in 1187 for BRIT and FRAN alike, and unclaimable in 1337 for BRIT.
Disadvantage: France is easier to claim, and a human player essentially gets a free king title in 1337. OTOH this counters Germany's free Bohemia....
I'll think on it a bit more.
What do you think of making Wales more difficult by adding the Cornish lands (Cornwall, Devon, Exeter) to it? That would eliminate the last free kingdom from the map by increasing Wales's size.