You can jazz up the diversity of clerics in your world without adding pages and pages of gods — if you allow for multiple (even conflicting!) belief systems about the deities you already have.
Deities in fantasy RPGs are often sketched out in rigid, one-note ways.
Imagine for a moment the typical campaign-next-door. Its storm god Thorzeus is Chaotic Good, his symbol is a hammer, and his clerics can draw from the domains of Tempest or War. If they thunk you with their hammers, you go deaf from thunder. The goddess of thieves, Lokhermes, is Chaotic Neutral, her symbol is the mask, and her clerics have access to the domains of Knowledge or Trickery. If they stick you with their daggers, you get woozy from poison.
If you go to the temple of Thorzeus, you find he has pretty much just one order of clerics and one kind of worshiper. All of the people in the temple seem to have fairly coherent, consistent ideas about who Thorzeus is and what he cares about. If you cross the street and enter the shadowed sanctum of the thief goddess Lokhermes, you find pretty much the same pattern. If you now hunt several cities looking for temples of those gods whose clerics think about the gods differently, you’re likely to come up empty-handed.
That lack of variety limits your options if you decide your character wants to be a cleric of Thorzeus.
“I want to carry lightning-bolt-shaped javelins and use those as my symbols. Do any clerics of Thorzeus use the javelin as a symbol, instead of the hammer?”
Uh, nope. Not in the campaign-next-door. You have a choice of hammer.
“Can my cleric of Lokhermes be a crusader who breaks thief worshipers out of prison and has the domain of War?”
In the campaign-next-door, that question is going to get noped before you finish the sentence.
If you follow up with “Well, okay, what other options do exist for that god?” you can expect more of that hammer. Thorzeus and Lokhermes don’t have options; they are the options. The gods are narrowly defined, and the clerics get narrowly defined right along with them.
But that kind of one-note divinity is weird. It’s not how gods have actually worked in the one realm we know of that’s real: Our own.
And that point brings me to my argument: If we complicate our fantasy gods in realistic, historically inspired ways, the whole game gets better.
The Historical Case
In the real world, one single god — the god of Abraham — has produced three major world religions that have sometimes warred against each other. That’s kind of interesting; in more than thirty years of gaming, I’ve never seen such a thing happen in a tabletop RPG.
But let’s drill down further. Within Islam, the schism between Sunni and Shia sects has often been bloody. ISIS right now is putting most of its violent efforts into the targeting of other Muslims. Same Allah, but different sects.
Christians haven’t done a lot better.
For just one incident, consider the St. Bartholomew’s Day Massacre of 1572. Protestants and Catholics, long at odds, came to Paris for the truce-making marriage of Princess Margot (Catholic) to Henri of Navarre (Protestant).
So far, so boring, right?
But the Catholics in power decided this was a golden opportunity to round up all of the Protestants who’d arrived for the wedding — and kill them. You can watch a film scene depicting this event, and much of its horror, in this clip from the brilliant film Queen Margot. (It’s in French, but blood and screams don’t need subtitles. By the way, the subtitled version of the full movie is worth a look for gamers. It’s based on a novel by the same guy who wrote The Three Musketeers, and given its poisonings, intrigues, sword fights, and battles, you can tell.)