I'd love to see that take on Divine spells. Perhaps have an open world game where your mana represents your favor with the God you're channeling. You replenish it not with items, but by doing things that please the God, as you said. Another neat idea would be to have the cost of divine spells go down significantly when wielding them against the god's sworn enemies. A cool idea would be to have you face off against a high priest of a rival god and then have both gods instantly grant their channelers with all of their powers and unlimited usage of them until the battle is over.
You could even have someone of multiple faiths who tries to balance his devotion to multiple gods so he can wield a variety of divine spells. It would provide for an interesting incentive to behave a certain way throughout the game, since many of your actions would either please or anger the many gods.