I think House Frey is a prime candidate for a mini-faction, along with Tully and Arryn. I would make Tyrell a full faction (can get them up to speed fast by just adding Tyrell version cards in a starter box for existing units - like they did for the neutral units when releasing House Bolton)...
As an avid fan of the books, I personally view most of the female characters as non-combat units; but, I can see how others might not. Definitely could have some with more control, movement, or token generating abilities. Maybe due to the Sand Snakes and Dorne also recognizing female...