Doing it over and over again will result in increasing numbers of variants. At some point the distinctions become meaningless, except for role-playing racism. The notion of tribe will also be in the process of erosion by the time of game launch, as crossbreeding will already have been occurring in the centers of political interest for generations.
I prefer to keep the notion of 12 tribes, with infinitely many possible variations. For those who want to split hereditary hairs (or heirs, for that matter), feel free to go for it, but realize that it's your choice and not the game's built-in lore or mechanics that are doing it.