I think this is an interesting premise for a story world, but like others have mentioned, what is the conflict? What is going to happen when the survivors meet these tribes - do all the tribes want to kill them? Will some of the survivors join these tribes, and vice versa? How will the survivors manage to live in this new world?
Like Lorenzo pointed out, there's many genre options that this premise alone could take the game; if you told us more about what the main conflicts were or even gave ideas as to how a player would play them out, then it'd be easier to answer your original question.