I actually really like the idea of a Holy Warrior/Paladin/Whatever.
Give them either the ability to preach or even a passive ability to influence commoners to join the faith.
I also think it would be cool if they had at least some of the Priestly abilities to calm the populace and stuff like that.
It would be a welcome addition to the religious game in BattleMaster I think.
Long term goals for it could be something like moral boosts in regions where the majority of people follow that religion, or even a boost to military units in battle if a Paladin of their faith is in battle with them. This would obviously be long term and hard to code, but the possibilities are impressive. And as I said, they would make the religion game much more interesting...
Note that due to the gamebreaking setup of having units moving between turn changes, I would not suggest having priests actually lead units. And trying to handle them as a hybrid class that could EITHER move as normal units OR as priests would be...a non-trivial task to code...and so something I wouldn't suggest.