I've been wanting to switch up clerical spellcasting for a while. Make it a bit more realistic and historical based on how real world religions believe miracles work. Make clerics different than wizards. Yet I also want to do this with the least amount of work possible, not because I'm lazy, but because I want to keep it as close to D&D as a I can. I'm not trying to make a new game here( I already did and have since abandoned it, I will probably put it up here this month), just tweaking the current game to get the flavour I want.
I came up with a system that I was satisfied with at first and then rejected. First of all cleric spells are rituals or prayers not "spells". So a cleric can perform any ritual that he is high enough level to cast. each ritual takes a number of turns equal to the spell level to perform. At the end of the performance the cleric makes a wisdom check to see if the ritual was successful. The check is modified by the following formula: 1d20 =< wisdom - spell level + (spells/day of the same spell level - 1). So a cleric with 13 wisdom performing a 1st level ritual who normally can cast 3 1st level spells/day would need to roll a 14 or lower: 13 -1 + (3-1) = 14.
There is also the easy solution that basically equates to mana points. the cleric has a number of prayer points equal to the total number of spells he can cast, and each prayer costs a number of points equal to the spell level.
I want to be able to continue to use the cleric spell list, while changing the method of cating to reflect clerical rituals; which means that limiting the prayers or rituals by a number per day are incompatible with my goals.
There are 3 factors I think are important for priestly powers: the proper performance of the ritual(holy symbol, holy oil, number of steps, etc), the will of the deity, and the faith of the priest. How to model those in game mechanics compatible with D&D is the problem.