There is a legitimate alternative to William the Conqueror, his name is Gui de brionne, actually OTL the Dukes of Anjou and Burgundy had legitimate rights to Normandy but did not forward their claims, what if Gui de Brionne married Matilda of Flanders and got support from the Duke of Flanders and got Normandy or alternatively William the Conqueror died, would Gui de Brionne try to invade England like William did or just ignore England completely?