Another option is to undermine the moral authority of the religion Francia follows, usually Catholic, and let them tear themselves apart with religious schisms. This can be problematic if you are yourself Catholic, unless you manage to snag Fraticelli, which is basically better than normal Catholic provided you can overthrow the old religion.
Assuming you're still House de Castille this isn't really an option, because you aren't particularly powerful enough or flexible enough in war declarations to snipe out holy sites.
I managed to become King of Navarra by accident when I had made a claim on the counties of Navarra and Alto Aragon and Alto Aragon had been taken over by Fraticelli (which later got nuked by the Pope sending an army of inquisitors). Then while I was letting my army build back up, my liege decided to declare war on Navarra for my claim on Navarra (I thought it was the county claim, not the kingdom claim), the war was quick, then right after we won, I spontaneously usurped the title of the Kingdom of Navarra.
I'm currently trying to either wait and let Asturias split into Asturias and Aragon or use the reconquista to Holy War for Toledo and get more territory and get stronger, also ally the kingdom of Portugal somehow.
Funny that you spoke of Fraticelli though since that had shown up (theres been an explosion of Cathar in the region though) and then got nuked by the Pope, so, that opportunity is gone.
