-The Eldar Craftworlds escape into the Webway and either start a new life there or escape to another galaxy or dimension. At some point Ynnead awakens and Slaanesh is vanquished.
-Humanity beats back the Tyranid onslaught by taking advantage of the psychic vulnerabilities of the hive mind.
-The Necrons lose momentum and are destroyed or forced into hibernation.
-The foolish and idealistic Tau are crushed by the Humans, the remaining Chaos disciples, or both.
-The Immortal God-Emperor is born anew and vanquishes the Chaos Gods.
-Many successful crusades are launched against Orks and other foul xenos.
-Ripley awakens from her milennia long slumber. Nids are now screwed.
-Tau are accidently stepped on by Gork (or possibly Mork)
-BTW, Eldrad (that DICK!) was OPTIMISTIC when he said that the Eldar would all die, cos THAT'S when the Ynnead dude awakes.