My general sense of the premise involves the question of when and how the mechanics enter into the picture.
For example, let's take the example of a Triton/Atlantean. I may give a player a list of the available ancestries or kin. There are no mechanics detailed. However, the player may see the Triton/Atlantean and think that it would be cool they could breathe underwater. Is that actually a mechanic in the game? Are there advantages or disadvantages to that? We are agnostic on these points at this time in the process. IF there are mechanics, it may vary based upon what game we are playing. D&D likes to provide detailed mechanics. You can breathe air and water. You have swim speed 30. You have darkvision. And so on...
But if we went to a game like Fate or Cortex Prime or Fabula Ultima... What then? There are no lists of races or ancestries in these games with mechanics to engage. In Fate or Fabula Ultima I may just have an aspect where I describe my character as "Triton Heir to the Throne of Atlantis." Nowhere in here is there a defined mechanic for waterbreathing. Instead, the GM asks the player what being a "Triton" involves, and if the player and GM agree that Tritons are amphibious, then they can breathe underwater as part of the fiction. But this would scarcely count as a mechanic in 5e D&D terms.