Before writing this theory, prepare yourself for spoilers for "First Shadow". Those who know the plot of First Shadow know that Vecna, like Will, was a host of the Mind Flayer since childhood. And you know that Henry's blood type changed after he visited Dimension X. And the rest of the test children were created using Henry's blood DNA. Does this mean that all the children received a piece of the Mind Flayer? And if so, then Eleven and Eight, and Henry, and Will will have to die for the Mind Flayer to die. But we also need to destroy the entire Upside Down and the entire Dimension X, and all the inhabitants of these worlds, including the bulk of the Mind Flayer particles.