The current system for war is "realm vs realm". There is nothing in place that would allow for something like a religious/guild type war.
I suspect that my idea would be a "coding nightmare" but I will bring it up anyway as it may lead to something that could be possible.
Under the current system, we have:
nobles join a realm
are placed in an army of that realm
are told that "*** (realm)" is their enemy
are sent to war to defend their realm or to defeat the other realms army(s).
The only real choice given to the player here is which realm they will join.
I will use Daishi/Mordok as an example.
What I see as being optimal here would be a choice of whether or not to join in a given war, regardless of realm.
Daishi has many followers in many realms, but currently there is no way for these "religious nobles" to band together to pursue any given goal.
It would be great if a "Priest of Daishi" (or a guild leader/member) could gather followers from various realms to band together in a common cause.
Not every noble of every realm that has Daishi followers, just the religious members that desire to see the "Priests" goals to fruition.
Great leader's gathering like minded followers from around the continent for a specific purpose.
Find a suitable cause with a suitable goal/enemy, gather warriors that believe in your cause, and go to war for a "just cause" that you believe in.
I think this could be much more satisfying than your ruler deciding what war(s) you will be involved in.
