Anyway, essentially a faction building for the holy brethren, one for each brotherhood, Teutons, Hospitallers, and Templars. Within which would be all the respective units and upgrades, and could include things from the priory (herbalism, call crusade, monkish stuff) if we wanted to get rid of that. Some things could in theory be siphoned out to the church - another factory that largely loses purpose once desired upgrades have been researched.
I have in mind something stronger than the average factories but weaker than a fort, so build cost might need to reflect that, probably with a battle aura for their respective defenders of the faith, and maybe to reflect their different flavours, ie boost to conversion for Teutons, boost to heal for the others.
A lot of details to be ironed out, but I'd like to know if this idea has any legs before investing too much thought into it. Could be interesting in conjunction with the recent enthusiasm for expanding the Muslim side of things.