So, say Anne of Brittany never becomes available - either because she follows a bunch of her (half-)siblings into an early grave, or because she gets married to someone who can actually defend her rights (like the king of England, as was planned) - I personally prefer the former. And Charles and Margarethe of Austria get married. What happens next? And, for arguments sake let's say that they actually have a few healthy kids - boys and girls - who survive infancy - Hell, her brother had all six of his survive, so I don't think a Gretl-Charles would be much different (esp since she and Charles are more distantly related than Philipp and Juana (their closest common ancestor being Joao I of Portugal; and Charles and Margarethe seem to be a bit further removed from one another - Philippe II of Burgundy being the closest one I can find).
How do things look for these half-Habsburg prince(sse)s of France?