2. Religious power- We are agreed a realm/religious connection needs to exist.
State religion- Do we agree a state religion "option" should be available (required for theocracies?)? That option could be a "text only" option. It's just "FYI." Or, it could be more significant, with "state religion" creating automatic religious tensions, or maybe restrictions on running for elections or getting lordships? Maybe grant auto-claims to regions for full members?
Unresolved issue- Exactly what effect should a state religion have?
Religious diplomacy- Connected to state religion but not necessarily identical: make realms suffer for alliances with religious opponents of dominant religions in domestic territories. Unresolved issue is mostly coding.
Apart from the restrictions on running for elections etc, I'm of the mind this would not only succeed in reinforcing the spirit of unity within a realm, but likewise add another twist when need work out your realms diplomatic strategies.
I also think the religious slant offers great scope for the creation of secular armies within the realm, ie although any noble can join it's bog standard army only followers of the state religion would be admitted to it's best, the holy orders.
