https://en.wikipedia.org/wiki/Eleanor,_Fair_Maid_of_Brittany#ChildhoodIn summer 1195, a marriage between her and Louis son of Philip II of France was suggested, for an alliance between Richard and Philip, but negotiations failed again. It is said[citation needed] that the Emperor opposed the marriage; and the failure was also a sign that Richard would replace Arthur as heir to England with his only living brother, John. This soon led to a sudden deterioration in relations between Richard and Philip.[4] Another marriage, with Duke Odo of Burgundy, may have been suggested, for in 1198 Philip ordered Odo not marry any relatives of Richard without his permission.[7]
From what I read, the marriage between Eleanor of Brittany and Eudes of Burgundy was prevented due to her prestige as the heiress of Arthur and also because Philip wanted her to marry his own son..
What if Matilda survives, she would not be a heiress of Arthur till Eleanor of Brittany dies so she can be used as a marriage pawn, who would she marry? Sancho of Navarre or Eudes of Burgundy, a marriage with Sancho would be arranged by Richard in order to further the alliance of England with Navarre, I think she will be the bride of Sancho rather than Constance of Toulouse, since Eudes of Burgundy is interested in marrying Eleanor there is a possibility of her being the Duchess of Burgundy and there is also a possibility of her marrying Raymond of Toulouse instead of her aunt, Joanna of England..