As for the priest, I can't really think of anything that it should be given, for the moment. It's underutilized not because of the class itself, but because of the religion game as a whole. Playing "guild-tender" is not for everyone.
Imo, to re-invigorate the religion game, the best approach would not be to make the priest class more alluring, but to democratize religion, by making it actually matter to non-priests. Religious options available to everyone and a religious subclass. Right now, typical knights can't really do much about religion, so how to make them care? But if the army could do more religious tasks, it'd be easier to use religion as a basis for foreign policy. Because right now, if heathens are next door, about the only thing the army can do is go destroy that realm. Alright, you can burn temples too, which granted is meaningful, but that just takes a few nobles to accomplish, it doesn't really occupy the whole army like takeovers do.
