In the more recent homebrew settings I worked, we tried to build justifications/rationalizations for any PC race into the settings.
Our 3e world had humans, Warforged (eventually), and 4 customs races (whose mechanics mirrored the standard PHB elf/halfling/dwarf/half-orc). But the setting was also littered with ancient magic portals to other realms so there was a ready in-setting reason for the inclusion of any race a player wanted, so long as they were okay with being a 'stranger in a strange land'.
Our 4e world was set in a port on the Astral Sea. It used the standard races, albeit with much of the standard fiction altered, but anyone & anything could 'wash up on shore/disembark from a mysterious ship' as a playable race.
Our 3e world had humans, Warforged (eventually), and 4 customs races (whose mechanics mirrored the standard PHB elf/halfling/dwarf/half-orc). But the setting was also littered with ancient magic portals to other realms so there was a ready in-setting reason for the inclusion of any race a player wanted, so long as they were okay with being a 'stranger in a strange land'.
Our 4e world was set in a port on the Astral Sea. It used the standard races, albeit with much of the standard fiction altered, but anyone & anything could 'wash up on shore/disembark from a mysterious ship' as a playable race.