So, England has Agnatic succession. I have become the biggest landholder in England and could pretty easily claim lordship over England... if I had a claim. How the heck do I get a claim if I can't marry into the family?
You can rebel for independence, then usurp the title. Or you can excommunicate the king, whiole having high piety and good realtions with the Pope, then he might ask you to take over the job. Or you could get a female heir yourself, marry prince number four matrilnearly, then press his claim and wait for the kids to inherit everything.
Just the first few methods that pop to mind.