In the Faerie Queene, the baddies hang out and are related.
It's as if all the people trying to stop you were part of the same facebook group that you couldn't see. As if they were all part of the same family, and some of them were gods.
Bad for reality, interesting for stories, very good for games.
There are two main ways this happens, different in each half.
In the first half some of the magical villains are related to Night, who is an old woman. If they lose a battle or get frustrated they can go to her in person and as for aid. We also get scenes where more 'human' or less mythic villains meet up in a soap-opera style, as if they were all part of the same shared universe that went on behind the scenes once the heroes had left
In the second half many of the creatures are more directly monstrous, they are often said to be descended from the same mother, 'Echidna', though she doesn't turn up on screen and they don't call on her for help. We also get a look at the renaissance versions of the actual Greek gods, Jove & Co.
D&D has toyed with aspects of this before, having all Dragons be descended from Tiamat, all Orcs from the main Orc god and all Dwarves from the Dwarf god, but they have never (I think. No doubt you will inform me if I am wrong) gone all the way with it, which is interesting.
THE MAIN IDEA
Linking monsters and monster races to a specific line of supernatural descent, in which living beings and species aren't just created by higher powers, but are children of specific higher powers, creates a cosmic-level meta-feudalism that would impact gameplay in an interesting way.
If everything is descended from everything else, then if you make allies or opponents of any particular being or species, it’s possible that they will essentially 'tell mummy', meaning that any significant action would mean offending, or pleasing, a specific meta-being.
And of course this would set off ripples of reaction in the meta-feudal structure. If you piss off one particular power by hurting its children then the same actions could please some other power opposed to the first, so you gain yourself an unexpected, conditional, and perhaps secret ally. Very much in the manner of Greek heroes or other Culture Heroes.
My imagined hierarchy for this system would go something a little like this;
LEVEL ONE - Prime Powers; Chaos, Time (these guys don't have stats)
LEVEL TWO - Literalised World Elements; Night, Sea, Sky, Sun, Fire, Death, Moon, Earth
LEVEL THREE -
A - Mega non-breeding monsters; Godzilla, The Kraken
B - Mega Monster Mothers; Tiamat, Behemoth, Shelob, Echidna.
C - Literalised Human Experience Elements; Panic, Fear, Love, Revenge, Thought, Craft, Harm, Thunder? (These might be considered Gods but read on to see why they may not be.)
A - Classic singular monsters, *The* Manticore, *The* Catoblepas, maybe *The* Peryton, possibly *The* Dracula.
B - Humanoid species; Orcs, Gnolls, Lizardmen, Dwarfs, Humans etc.
C - Non-Humanoid animalistic monster species.
The idea here is that for everything that either wants to kill the PC's or that is simply in their way, that particular thing, or that species, has a specific mother and a specific father.
Now that mother and father might not necessarily be sympathetic to their children, they might be total dicks, or they could be giant monster mothers with (as is traditional) uncounted young. So if there is only one Manticore, and its literally the child of Spite itself shagging Shelob, then neither Spite nor Shelob might actually deign to notice that you killed or assaulted their child.
However, it’s a quality of complete dicks that they will often take more offence than they actually feel. So if you kill Spites child, even if Spite didn't really like them then they might come after you anyway.
And even giant monster mothers might notice if you start hewing through a large number of their young, and D&D heroes tend to do that.
And of course, Thought hates Spite, because Spite was such a dick all those times, so if you piss of Spite, then Thought might throw you the occasional low-key assist.
Add to that, the Greek-style conceptions due to cheating; if Thunder has been boning Air behind Earths back, and Air gave birth to a Super-Eagle, then Thunder may well want to protect it, but Earth will hate Thunders illegitimate children. So to rob the Super Eagle of its powers, bring it to earth.
And of course that old Greek and Renaissance standby, the child of rape. A child that might be protected by or hated by, one or more of its parents, depending on how they feel about it.
Constructing an actual monster-Power identity family tree would actually be pretty difficult, if you wanted to do it all in one go. There might be procedural ways to do it? Let me know in a comment or something if you want a method and I will give it a go.
WHAT KIND OF GAME WOULD THIS ACTUALLY WORK IN?
A mythic 'original heroes' situation in which the PC's are the very first quasi-superheoric culture heroes to take on these monstrous or primal powers that threaten humanity, and win.
Myth, symbol, nature and eternity would inter-penetrate in a very material way . You could walk to the edge of the earth because it hasn't been sphered yet, reach the sky if you climbed far enough or find the palaces of Dream and Death beneath the ground. A place where, if you talk to the moon, or the sea, or the night, there is a small, conditional chance that they will turn round and talk back.
I'm imagining a Neolithic level of technology, a culture without religion as we understand it, where humanity (or just 'the people' really, the 50 or so people you know to be people) are always fleeing the ghosts and zombies of its own dead because no-one knows how to lay them to rest (you need to invent religion by talking to the Abrahamic Sky God or literally going to visit Death yourself).
A culture without fire (you have to steal it), possibly without sin, it could be that one of the PC's is the first to actually kill a family member and the sky god is so pissed he brands them and releases the harpies himself (maybe the first harpies ever). Probably you can talk to the animals, and the wind, or you can if you have a language skill, because the Sky God hasn't got pissed and 'Babel'd' things yet.
And of course, if the PC's manage to murder or overturn the higher powers then it turns out that actually those dudes were the Titans, and the PC's were the New Gods all along, and what they are up to is actually the mythology of a new world.
If you were to meet other humans then you might not realise that they *are* human. You know what 'the people' look and sound like - they look and sound like you and your kin group - but these things look different, sound different and smell different. you might not have the physical context to be certain what it person, spirit, other or monster. The idea of 'human' is a concept you would need to invent.
The idea of not being able to conceptualise something until you find it in the world is a problem - especially as it relates to the crafty problem solving at the heart of D&D. Not being able to smelt metal until you go and talk directly to Craft sounds reasonable, but what are the borders of what you can and can't invent? Can you invent archery? Levers? Beekeeping?
I would probably run it as if years passed between sessions and the PC's gradually aged, with degradation rules for very old age. Wouldn't kill them between sessions but death would become more and more likely as their increasing status would mean there is more and more stuff they would have to do.
Possibly hack the tribe rules from Wolf Packs and Winter Snow.
Allow PC's to breed, if they make a kid then it gets a mix of their stats and becomes a possible future PC.
PC starting ages begin around 14-15, people not expected to live much past late 30's. You can start older and the advantage is that the oldest person in the tribe has a dominant status, everyone respects age, of course you will die sooner.
Classes would be very basic 5E; Fighter, barbarian, rogue, with the magic stripped out or stripped down.
Races; preferably everyone is human and part of the same immediate kin group. Could maybe allow skins of the different races as half-breeds with human sub-species; Neanderthals/Dwarves, Halflings/Homo Florensis, Half-Orcs/mum shagged a magic animal, Elves/mum shagged a magic tree.
Treasure would be knowledge and food. If you discover a sustained means of increased food production then your tribe increases, and of course you are the legendary figure that invented that (or stole it from the powers).