Uh....... don't think you can become a member of the Great Khans, let alone the leader of them
You actually can. You must help Dead Sea defeat Forlorn Hope.
its not just that those, although the wikia says its just that, but i think you need a negative reputation with the NRC AND to have done that, or maybe not to have completeled some NRC quests and to have taken Forlorn Hope. as the first time i did it by accident but everytime ive tried since it hasnt worked so clearly its not just defeating forlorn hope and being idolized by the khans that does it but other things as well. But no, it doesnt seem to make any difference to the ending cutscene, but then most of the big things dont.