So there are some duchy's and kingdoms that have both mountain (dwarf/goblin only) and normal tiles within their borders. What I am suggesting is the option to invite colonists from a race that can colonize areas you can not as long as it falls within your de jur title. Like if I am a Human or an elf and there are mountain tiles that fall inside my dutchy I should be able to invite dwarfs to settle those lands. Or if your a Human that is sided with Sauron you could invite goblins to settle those lands and so on.