Well we know that the Golden Throne is failing, so sometime within the next millennium it will finally give out and the Emperor's current body will die. Then we see him get reborn through the Warp as a full fledged God, unlock the Webway, do away with the Astronomicon leaving the Nids aimless, and just kick ass all around.
That's of course only one scenario, albeit the best one, humanity considered. Then there, is of course, Ynnead. Whose birth will be of such absurd potency that it'll tear an entire new dimension into the universe, and give Chaos the beating it's had coming for aeons.
How about Eldrad's Spirit Stone actually survived and he somehow escapes from the realm of chaos then boom! Eldrad dick powers fix the Galaxy/fuck it up more but only for the younger races so its cool.
