Mike needs to finally finish his campaign...
aka how the DnD universe perfectly reflects the in-show's universe, through a nested universe lens. A story within a story within a story...
As controversial as the nested universes theory is, I want to explain that there are many hints towards it. The basis of the theory itself is based off the DnD universe created within the show.
In the very first episode, Karen cuts Mike's campaign short.
Will rolled a 7, which meant the Demogorgon got him. Once Will revealed this to Mike, that sealed his fate. That's where the campaign ended. It basically ended with Will's character dying.
At the end of the season, Mike turns back the clock allowing Will's character to survive his fight against the Demogorgon. He rolled a 14 instead of a 7!
Basically after Will survives, Mike RUSHES to finish his campaign to the confusion of the other boys.
So as much as Mike wants to end the campaign there, ensuring Will's character's survival, he can't. There's still more story to tell.
The campaign finally continues in ST3, with Will leading it this time. Mike says "it's so early" because he doesn't want to continue the story just yet. Within the campaign, there's a ton of foreshadowing (that's for another post) but basically Mike rushes again to finish it.
This time, Mike gives up and allows their characters to all die. Will becomes very upset by this. Mike later deeply regrets it...
Mike regrets it because Will donated his DnD set, meaning... he was done contributing to the campaign. They sacrificed themselves and that was the end of the story. Mike absolutely does not want that to be the end! So what does he do?
He turns back the clock again... and continues the story for Will, but without Will.
Mike wants to go back to DnD to properly finish the campaign but is held back again... this time by Lucas who is unable to play with them.
This was why he is greatly affected by Lucas' basketball game. He was held back from finishing the story properly.
Erica (the recipient of Will's DnD set) continues the campaign with them.
They all nearly died again... by Vecna. But two characters were left and they decided to stand up against him.
The odds were not in the favour, but despite that...
Mike successfully changed the story in time.
The DnD campaigns are a clear reflection of the story within the show itself. A story within a story.
So... what if there's another layer to this?
What I'm arguing is that this may be a major hint that a Mike exists beyond the show and is the one writing the story of the show itself. A story within a story within a story.
Will originally was writing it with him, but he moved west (click the link for the symbolic definition of that... my apologies in advance... ), unable to complete it. Mike has to complete it on his own...
He changed the tragic ending to a happy one. But he must complete the story in time before it's too late. Full theory post on the details -> here. (Warning it's tragic and dark).