I have a couple that I think are neat:

In my setting, there is a race of people called Dantalians. They’re your typical red-skinned bat-winged small-horned spade-tailed demony-looking demons. (My main character’s sidekick/familiar is one.) They feed on life energy through skin contact. When they’re young (like said sidekick) they are irresistibly adorable because they literally feed on cuddles and it’s part of their hunting strategy. It’s hard to look at a juvenile Dantalian and not want to hug them. As adults they require increasingly prolonged or increasingly intimate contact to get enough to sustain themselves.

Most Dantalians are incredibly careful with and considerate of their partners and never take enough energy to actually hurt them. Consent is a very serious cultural thing for them. Those that are not that way – the ones that either don’t care if they hurt the people they feed from or actively seek to do so – are called succubi.

To call a Dantalian a Succubus is a serious insult; to call her that to her face is fighting words.

The outer planes that Demons come from are all named after a certain vice. There are nine of them (the seven deadly sins plus Ignorance and … for some reason I can’t remember the other; I’ll have to go look it up). Ignorance is made up of pieces broken off eons ago when Pride and Sloth suffered a planar collision, and the one I’m forgetting comes from something like that too.

Each of these outer planes gives rise to certain types of demons, who proliferate into the various Known Worlds. Dantalians, for example, are originally Lust Demons even if they are a bit far removed from their homeworld now.

There are no Greed Demons in the Known Worlds. Greed doesn’t allow anything to leave its domain. The Demiplane of Greed is essentially a black hole.

For context, the world I’m working on is primarily used for D&D and Pathfinder campaigns.

I think my favorite ones are either:

• Dragons aren’t the progenitors to the dragon-shaped-humanoid race (dragonborn, to use the D&D term; vorelthyr to use the term I made up to be lawyer friendly regarding the OGL, etc), but rather the other way around. Dragons, kobolds, and lizardfolk all share the same origin, give or take. They’re all effectively just fleshy robots; kobolds were made for basic tasks, lizardfolk were short-lived footsoldiers, and dragons were living siege weapons. This is literally ancient history and unknown to most folks, though, given that the primary home of the vorelthyr dropped a magical nuke on itself a few thousand years back.

• Humans, and all mortals really, were a complete mistake that just spawned out of a one-in-a-billion chance. When the powers that be (cosmic entities) were shaping the world for whatever undefined purpose, they never intended mortals to exist. And it was such a mistake that a good 90% of the prehistoric humans were wiped out, because the cosmic entities decided ‘eh, we can sink this continent and start over’.

I am also using a D&Desque setup for my world (currently leaning towards Pathfinder 2E).

I am fully embracing both raise dead and reincarnate spell effects. This has the implication that as long as someone is rich and take precautions, they are hard to kill for good. “Raise dead” will bring them back until they reach old age, and “Reincarnate” will bring them back even if they die of old age - but with a young, healthy body from a random ancestry!

This has a number of interesting social effects:

• Unlike in our world, bigotry against people who “look different” is not tolerated by the ultra-rich, since they might very well find themselves within a body of a marginalized minority the next time around!

• Wealth is no longer “dynastic” (though this is still an ongoing shift in my world). Since the ultra-rich generally hope to stave off death indefinitely, they see less reason to build up “heirs”. They might still have large number of descendants (often from different ancestries!), and might even be fond of them… but often, they are seen as means to an end, as (presumably) loyal servants for their financial empires.

• There is a sharp divide between those who can afford those repeated resurrections, and those who are well-off but who still can’t easily afford coming back from the dead. The competition among the latter who wish to join the former is fierce.