one really old idea I had which I know is obscure, is an Uncharted 5 that has a different protagonist -
this one moment in a cutscene in Uc2 where Nathan is looking at resistance soldiers walking by, one of those soldiers could be looking right back at him and that would be the new main character.
the setting of having an ongoing civil war in Nepal was always something that sparked my interest regarding Uc2, but it was only a backdrop in the story and never really mattered. but that one Nepali foot soldier could have his own "Uncharted" story, I think. it would make a hypothetical Uc5 an anthropology but still in the same universe.
I don't expect ND to ever do this hypothetical game since they're done with the IP, but if Sony ever even did something like this, it's another project I'd expect Bend to do since they have experience with it.