Ease is what you want. There is no mileage in making settings massively complicated. There are no real world examples of different sentient species coexisting peacefully. Humans aren't even capable of coexisting with their own species.
Personally, I don’t think having a crashed spaceship would be very different from most D&D settings. I would go full Metamorphosis Alpha, and make the spaceship the setting. But not let the players know until they find a mysterious hatch.
“The world is hollow and I have touched the sky.”
I overheard a cover of someone singing something from Muppet Christmas Carol. Don't know who the band was, but I'm surprised there isn't more of this. I guess Disney ruthlessly hunts down licence violators?
Heard some choral renditions of traditional carols.
So far no Wham!
The "very small numbers" approach works better than removing breath weapons. But damage resistance makes for a good plot point. There are several Dragonborn in RotFM because they have cold resistance, and the climate suits them. If a character has fire resistance, they are likely to be drawn to...
A setting needs to have some sort of centerism, human or otherwise. There will always be a dominant species. If there are others, they exist in small numbers, and define themselves relative to the dominant species.
No it isn’t. I’ve told you before that halflings were the most popular PC race in my group, outnumbering humans. That was a few years ago though, and since then the halflings have themselves been outnumbered by warforged.