We consider race and culture two sides of the same coin actually. This is obviously not correct in the real world, but for a fantasy game it is very appropiate. We have no need for the rare dwarf that was raised among forest people
I want exactly that possibility. Granted, it's very easy for me to do this myself within the system, but I see no reason why you couldn't separate race from culture. It's not going to mess anything up to any large extent.
That way you put out a core system that has: Humans, Elves, Dwarves, Hobbits, Gnomes.
Then you put out a largish single book or multiple books that sub-divide each race into sub-races.
Humans: Hillman, Mountain Man, Mariner, Ruralman, Urbanman, Plainsman
Elves: High, Wood, Grey, Underground
Dwarves: Mountain, Hill, Plains
...and so on.
Then you have optional rules for what to do when a Wood Elf is raised by Mountain Dwarves. i.e. Culture, Background, Adolescence, whatever.