I was imagining a setting where all fantasy races are essentially "human" (but not really, see below), instead of different species. Some inspirations include Shadowrun, X-Men, the Graygem gem in Dragonlance, and Sweet Tooth TV show.
The logic:
- The concept of elves, dwarves, orcs, etc. are basically humans with cosmetic changes
 
- Differences between races is even less than it used to be with the cultural traits separated out
 
- So just call them what they are: they're all the same race, just with different magically-induced traits
 
The fiction:
- One day, to everyone's shock, every race start giving birth to children that are randomly of any race
 
- People originally think it's wild magic or something
 
- But it turns out that the original mortal race were shapeshifters at birth (a magical version of Darwinian adaptation?)
 
- In ages past, for some reason, these shapeshifters got locked into distinct forms that stopped "evolving" at birth
 
- Now for some reason, it's all been unraveled
 
The bad part:
- It's a little bit too on the nose
 
- It totally messes with most campaign world-building