My brother has been DMing for a long time — around 30 years, if my memory is right. One thing he’s been pretty worried about lately is the place of gods in the game universe. He’s felt for some time that it doesn’t make a lot of sense for anyone to worship an evil god. Pretty much every pantheon has several evil gods. Moreover, if every god can work miracles, why should anyone worship just one of them? He’d rather just take the old “implied Christianity” of D&D to use a stand-in Church of a creator, with one god. I’ve pushed him to make room for Druids, which are nature-worshipers or perhaps pagans in his games but he like to leave the Church ascendant in most civilized areas. He definitely doesn’t like the idea of introducing multiple conflicting pantheons, as some D&D campaigns did back in the days of Gods, demigods & heroes and Deities & Demigods. (In fairness I think both supplements instruct DMs to choose one pantheon, but there has always been an anything & everything goes mentality among a lot of players!)
Anyway, in an effort to mitigate some of his concerns I suggested that the gods might be called various names but all actually represent a small set of actual divinities. We both prefer historical pantheons, so I created a table of corresponding gods & goddesses, allowing a multitude of cultures and cults while keeping the divinities down to about a dozen individuals. I had it in my mind that any gods from each pantheon that are not covered should be equated with demons, devils, aliens, or other less-than-divine beings, perhaps demigods (which could still exist as distinct entities in their multitudes, but which would in principle be mortal). A pdf of the chart is here: The universal pantheon. I tried to correlate Greek, Roman, Egyptian, Norse, Celtic, Hindu, and Mesopotamian gods into the list, as well as Christian saints and the Trinity. The Mesopotamian gods are combined from the Sumerian, Babylonian, and Syrian pantheons — I just used the names I was most familiar with.
I don’t think this idea is really all that original; I suppose occultists have attempted such correspondences for centuries, and the Greeks and Romans famously identified their gods with the gods of other cultures.
I based my chart partly on the Interpretatio graeca linked above, and partly on some work I’d done for my GURPS Bestiary of Spirits, as well as additional research. The infamous list of saints from Bruce Galloway’s Fantasy Wargaming certainly helped too!
Although I have problems with the conceit of clerics just worshiping one god in a pantheon, this system would allow such dedicated clerics to recognize their own god as another when in a different setting. But mainly it would give a DM some rationale for why the northerners worship Odin and the southerners worship Zeus and the two gods haven’t smitten each other’s followers.
We have not actually used this idea in a game, but I though I’d share.