So Noah had this to say about season 5
“I can just tell you that I'm very very excited for what's to come. I think they did a great job with Will's character this season, and beautifully addressed everything they needed to. The way they closed the show is just perfect — the story started with Will, and it’ll end with Will.”
See the final line?
My theories/predictions are 1) That the story will end with young Will safely coming home and nothing happening to him. For them to achieve that, El will sacrifice herself and lock herself in the Upside-Down world. She will stay there closing off all the gates and making it like none of this happened. If you remember time in the Upside-Down world is stuck on the day Will was abducted. I think that's a big hint. So imo there is a possibility that what I said previously happens.
2) Another idea might be that they will travel in time to undo the mistakes they did. So they beat Vecna finally and put a stop to him. They might suffer some consequences but no idea what it might be. So a happy ending I guess?
But I think the first one is more in line with the vibe of Stranger Things.