looking for ideas on character creation.

[Maho-Tsukai]

If you said that he's faithful and also would take a class that leads to a lot of powers, I suggest a cleric. You don't have to be CoDZilla to be powerful as a cleric either. However, cleric as a class may not fit his backstory, since a cleric would never challenge his own deity. Perhaps he challenged a deity who was apposed to his patron in some way? If cleric dose not work for your fluff, then wizard works fine as well. While you have an arcanist in the party already, you said he/she was a gish, so a truly dedicated, controlling, batman wizard would be would be a welcome addition to the party, A wizard will eventually gain great power as well, being a tier 1 class like the cleric. Also a wizard fits the fluff since arrogant arcanists with great powers who want to become gods is such a cliche(thanks to final fantasy, at least in my mind.) that one of the D&D gods has this as his origin story. You of course know who I mean...the guy missing his hand and eye, Vecna. He started out as a mage and eventually wanted to become a god but instead of failing like most who try this he actually succeeded. So wizard fits the fluff well.