Eleven's despair over losing to Vecna in Stranger Things season 4 is sure to make the villain even more powerful, and Will more vital, in season 5.
"Kas died via bat bites, like Eddie, but was resurrected as a half-vampire to eventually defeat Vecna himself. He also wielded a sword and shield, like Eddie, and they thought Vecna was defeated until Will said that he still felt him JUST LIKE IN THE CAMPAIGN. The Duffers don’t add details like that for no reason. Plus, I’m just holding out hope that Eddie comes back somehow. I refuse to believe that they just left him in the Upside Down, but who knows."
In Stranger Things season 5, Vecna will most likely take advantage of Eleven's anger, despondency, and depression to disempower her further. The fact that Eleven could resurrect Max in the Stranger Things season 4 finale proves her powers are still present and may even be stronger than ever before.