Although essentially a fictional narrative, Cornwell's Arthur series does go into the conflicts between the Christian and Pagan kingdoms within Britain itself, as well as your typical political skullduggery culminating in raids and border skirmishes. I'm really happy that you've made room for this type of internecine confict in the game.
I would love to play scenario based on the Brythonic diaspora into Gallaecia, with a dramatic defensive retreat from Saxon or Irish forces as civilians are herded into ships to sail into new lands.