here is my thinking on clerics, and how they fit into a system.
my base assumptions:
1. adventurers are not normal people. a fighter is not a normal soldier--if you've been in TTRPGs for a while you've probably heard *someone* explain why you can't send an army into a dungeon. A thief is not a normal petty criminal. wizards are not normal, period.
therefore, clerics are not normal religious figures, and the normal religious figures of your game should not be able to do the things clerics are able to do.
2. clerics are not wizards. wizards should not be able to do the things clerics are, and vice versa. even if the wizards can cast healing spells, and the clerics can call down lightning, the mechanical and fictional basis, methods, and results of their actions should differ.
so wizards can heal, but for a different cost, with a different result, and fictionally, in a different way.
based on these, my main point:
clerics are defined by their Relationship with Outside Forces. that their power is not within them is important. that it is a relationship is important.
what does this mean.
for example, the Skerples sorcerer is defined by the Instability of their Internal Power. mechanically, there is no resource--the sorcerer is a game of betting on a jenga tower.
GLOG wizards in general are defined by the Management of a Resource. how much of their resource they must expend to succeed, while not burning through it all too fast--even flavorwise, if you subscribe to the spells are living things business, there's no relationship there. you're looking for a (living) resource and figuring out where to stick it, which ones to shove in your head and which ones to shove in a scroll.
So. now you've got a choice. is your cleric magic or not.
if the cleric is magic, they carefully maintain a relationship with an outside force (or forces) who in turn channel their functionally infinite power through the cleric. what limits the cleric is not how much juice they personally have to power workings, but how much juice the outside force is willing to provide. huge and powerful magics is not a matter of cost to the cleric--they won't burn up in smoke because the power of a miracle was too much for their mortal body to handle. it's whether they can persuade the outside force to manifest, to do them this favor.
and the danger is in the unpredictable machinations of these outside forces. amaterasu hides in the cave, God turns Lot's wife into a pile of salt, Poseidon blows Odysseus off course for a decade.
this is tricky to represent mechanically in a satisfying way, i think. at least, nothing comes to mind immediately. i need to think more on this kind of relationship system, because the current model of "here's some tenets, follow them and receive boon, break them and receive punishment" isn't doing it for me.
if the cleric is not magic, their power is in their ability to create relationships with outside forces, which is not possible for the other classes. exorcist is the all time classic. using nothing but some paper with stuff written on it, some candles, some beads and their unshakeable faith, the exorcist converses, bargains, tames all manner of otherworldly powers.
that's a bit unclear, so lemme explain another way. with the classic 4-man party (fighter, thief, wizard, cleric), what does each one deal with? (my thoughts here are heavily based on this wonderful post that i love) the fighter removes hostile combatants. the thief removes non-sentient obstacles. the wizard deals in magical problems.
then the cleric deals with the things the other three don't. here's a fun chart i just made.
the non-magical cleric's power either comes from the ability, the know-how to command, or to deter, these sentient, ethereal forces. (i feel like there's a better word pair than material-ethereal, but i can't think of one)
that's my thoughts on clerics. course, you could always just not have clerics. fighter-thief-wizard is a great dichotomy.
now that i'm thinking about it, i should write something up on fighter-thief-wizard vs. fighter-thief-wizard-cleric
ADDENDUM: had some more thoughts over on the GLOG discord, here's a bit from that
zombies, wights, skeletons, etc--ran as they most commonly are (basic enemies), the fighter is actually the best bet to deal with these. undead enemies are commonly a test of how much damage a party can pump out, and that is the fighter's specialty.
(you could make undead enemies the clerics purview, that would probably involve something similar to 5e's famous channel divinity: turn undead)
magical traps, mysterious runes, rooms with wacky effects--that's wizards, who often have mechanics that allow them to identify unknown effects and disable magical fixtures
magical enemies, say... sorcerous pigman, fire breathing wing-boars, manticores? again, wizards are the ones identifying these creatures and protecting the party from their strangeness
spiritual and ghostly hijinks:
- possession, confusion, mind altering effects
- weird ghost stuff ("save or the ghost enters your body. become increasingly hungry. no amount of rations can sate you! do something about it or die of hunger in a day. the ghost died of hunger" if you can be exorcised, this challenge becomes trivially easy)
- angry spirit (intangible) chucking shit around the room. fireballs don't hit intangible things, that makes no sense.
- non-harmful obstacles ("you get a deep feeling of dread as you enter this room. cleric, you know this is a spirit at work. your hirelings refuse to enter. deal with it or find another way around")
so i guess what wizards specialize in is magical problems that can be solved if you know what the problem is
and what clerics specialize in is magical problems that can't be solved even if you know what the problem is.
just like how thieves deal with mundane problems that can be solved if you know what the problem is (traps, that is--im leaving out locked doors and walls to climb)
and fighters deal with, you get the idea.