I think we're definitely heading for a major reckoning between Eleven and Vecna. Eleven is going to be dealing with her guilt about freeing him, and Vecna is going to want to enact his revenge on her.
Personally, I think we might see Eleven transform into some kind of Vecna-like character, perhaps trapping herself in the Upside Down in an effort to finally destroy Vecna and close the portal. I think it's possible that Eleven will turn into an evil or destructive Upside Down entity in the process, and her friends will have to kill her in order to save themselves and the town. (Of course, I doubt she'll actually be dead, but I'd wager the season will end on a cliffhanger.) Or maybe they'll have to trap her in the Upside Down.
I can imagine season five beginning with a possibly evil or questionably human version of Eleven sending electric signals from the Upside Down to her friends in Hawkins. — Eden Gordon, assistant editor, celebrity and entertainment