Hello. I've been thinking of something to fill in one kinda missing piece to Thrones of Britainia and that would be a religion system.
The English, Welsh, and Gaelic factions are Christian nations following the ways of the church and doing whatever they say.
The Great Viking Army and Viking Sea Kings factions are nordic factions that believe in the different gods of Norse mythology.
I suggest that the various important churches in the Christian religion provide different bonuses to whoever controls them and the Norse religion builds their shrines to the Gods.
The main effect of the Christion factions is to stop the "Pagan" Norse from converting there followers. So giving bonuses from the Church hierarchy to your Christian factions for destroying "Pagan" shrines and factions.
The main effect of the Norse factions is to keep the fight going and to always remember home. Celebrating the various Gods and Goddesses and building shrines to help with your culture be it for combat and/or the betterment of your people.
Where the paths intersect. The Norse could take over more important churches and make them an epicenter for a major God or Goddes giving bonuses to the entire faction you play as such as better combat bonuses and/or agricultural bonuses and/or alligences of not only the civilian populations but of generals and govenor. The Christian paths are to keep your civilian and general/govenor populous under the control of the church either by hosting events like parties for royalty or giving food or money to the populous.
The Sea Kings are new to the isles after just leaving home so their converstion to the Christion faith will be difficult to change their peoples ways.
The Viking Army have been on the isles a little longer so they have a responsibility to their peoples to keep them safe. They could be converted to the Christian faith or they may choose willingly for better bonuses from Here King English but they don't want to lose their ways of home. Keeping the "Old Ways" grants them bonuses with other Norse followers and vice versa for the Christian faith.