Oh yes, when I said "them", I meant the Dukes of Brittany actually, not the Valois themselves.Marrying the Valois when they lost all of France would probably prove detrimental to whichever family decides to do so as it doesn't give you any land and would make you an eternal enemy of the Angevin.
You're right, though if the Dukes of Brittany are looking for a marital alliance with the Angevins, I think they'll choose someone far enough in the line of succession.Edit: Angevin have to assert their control over France, trying to conquer other nations would be overextending as you would have to a) fight a war b) fear intervention from Scotland c) put down the revolts from the French nobility and maybe even in England.