Philippe Auguste seems to reserve Eleanor to his son, what if Philippe Auguste has Eleanor of Brittany married to himself and his younger daughter to Arthur of Brittany and his son has his OTL wife, what would be the consequence of these marriages.
I think the marriage between Philip and Eleanor would happen before Richard I dies just after the death of the fiance of Eleanor of Brittany.It's an interesting scenario especially if Eleanor gives birth to a child, especially a son. Does that son inherit Brittany or even have a shot at the other Angevin lands? Do the English barrons turn to Philip Augustus or his son Louis as both are married to granddaughters of Henry II?
I looked up the proposed marriage between Eleanor of Brittany and Louis VIII and apparently it seems she was against the match and that it Richard I arranged it with the inattention of sidelining Arthur in favour of John. IMO, the opposite would have happened and John is very lucky that it didn't go through.
I think John would be careful in dealing with Arthur ITTL.Richard might go to greater lengths to secure a succession from his own person; the descent from Eleanor would definitely serve to legitimize the inevitable French conquest of the greater part of the Angevin Empire and most likely speed up the annexation of Brittany into the royal demesne.