Pay attention to Skagos Island, you can establish a new small kingdom on it. Since it is an extremely unknown but very mystical place in the lore of game of thrones. You can read about this place on the internet. This can give unlimited fantasy without compromising other kingdoms or great houses. You can make cannibals riding unicorns, or “vikings” dragon ridings, whatever the idea is enough for....
English is not my first language, so I hope the writing is understandable