Hate to do this to you again but... depends on when she marries him. Before the civil war? Before the nobles rise up against Enrique in favor of Isabella's brother? Before Joanna marries Alfonso of Portugal? Still how might it happen? Ugh, Isbaella is 30 years younger than Louis XI but 20 years older than Charles VIII. Louis I guess, might have offered his brother Charles the Duke of Aquitaine at one point (who was only 6 years older). Say Isabella goes for it, it works, Charles somehow manages to become King and Louis dies early. Then there will be no marriage attempt at Burgundy. Considering what happened to Charles (TB and an STD)....
Assuming Isabella's brother is still killed, would Isabella even be Queen of Castille, or would it be Joanna Beltraneja? There are no other candidates except Isabella herself but if she's already in France? Still, Isabella the Catholic as the Queen of Castille AND the Dowager Queen of France by age 25? Assuming she retains power in France (I have no idea, Isabella was pretty sharp) her attention would probably be directed westwards leaving Burgundy alone. Aragon is going to be in a lot of trouble even if they totally capitulate on the Rousillon.
Would we see interlocking alliances to restrain France-Castille?