Okay... From what I learned, somewhere around the fall of Roman Empires, Brithonic lords send warriors to what is now Brittany, France, seizing lands (and if Wikipedia is true, a part of what is called 'Green Spain', by the Basques Lands). MAYBE there was peoples who came from there before, but...
So, and correct me if I am wrong up in the premise...
What if they never came? Maybe by settling away, elsewhere, like ireland, or the 'Green Spain' (Galicia?), but not on modern french ground.
Would it change much the history of France? Who would live in Brittany - heck, how it would be called? Armorique? What would peoples speak there - an alternate form of this romance language of the 'Oil' familly, Gallo?