If Henry VI marries earlier and quickly produces a male heir (or preferably an heir and a spare and some daughters) his reign may be seen as much more stable…
Some matches
-There's no Burgundian match, since Philip the Good didn't have a daughter the right age, but his nieces the Cleves girls might have had a distant shot, Agnes or Helen were the right age,
-Frederick III's sisters may have a shot especially after he's crowned HRE in 1440, Catherine of Austria was close enough in age
-Another interesting possibility are the older half sisters of Ferdinand of Aragon, Blanche and Eleanor of Navarre, both are very close in age to Henry VI, and the oldest will become an heiress to Navarre after their brother dies, and Aragon has had its conflicts with France
-No candidates in Castille
-There not too many girls amongst Portuguese infantas either, pretty distantly in the royal line there's the mentally ill Isabella of Portugal the OTL mother of Isabella of Castile or her sister Beatrice of Portugal the OTL mother of Manuel I of Portugal
None of these options are really great. The pickings are pretty slim.