Before I begin my theory, I want to disclaim: this theory is based on the Mind Flayer being the final villain, and it will feature heavily in Byler, so people who are against Byler and don't consider the Mind Flayer a major villain will find this post extremely uninteresting.

So, I have a theory that Eleven and Hopper will fight 001, and as the fight progresses, Vecna ​​will lose. When 001 realizes he's going to die, he'll show Eleven a vision of Mike confessing his love to Will (the vision will be true). This will break Eleven, and in her anger, she'll finish off Vecna ​​and absorb his powers. And so, victory seems assured, but the Mind Flayer appears in its true form, a 150-meter-tall octopus-like creature, and kills Hopper. Jane is completely broken; she no longer wants to live, her only desire is to kill the Mind Flayer. She forces the entire massive mass of particles into the UpsideDown, and the portal closes forever, but Eleven can't handle the strain and stress and dies.