It's been 2 years. Mike & Will are 17 now, and things are going better overall–or how much they can for an apocalypse.
The stranger factor is the fact that nothing has gone wrong yet. Or are they too late to guess his new plan?
1) campaigns aren't held in mike's basement anymore
this is actually a fairly big deal in the first episode. the conflict with d&d and lucas' game wouldn't have been an issue if it was just up to mike and dustin, since they clearly wanted to support lucas. and if mike was still DMing at home then karen wouldn't have needed to give him a curfew at all, she could've just planned to kick the others out when she thought it was getting too late like she did in the first season
we know from mike's later breakdowns that he needs to feel needed and doesn't think he is anymore. the main reason the party hangs out at this point being outsourced to the hellfire club instead of him probably contributed to that
2) karen is pleasantly surprised to see the party
she never used to feel the need to encourage them to come over like this - even with ted being grouchy about it - because mike's the one with the large friend group (as opposed to nancy who only ever has one person at a time), and his house seems to be the default hangout spot, even when he's not there like in season 4. karen's reaction implies that mike hasn't been having people over as much (if at all)
3) the basement is a mess
the floor space isn't clear anymore. the curtains are closed. the lighting's washed out where it used to be warm. the slight change in furniture arrangement, on top of all the clutter (blankets and pillows, a ton of games, dishes and cutlery on the side, junk food boxes; implying that mike's been eating, sleeping, and generally spending a lot of time in the basement), makes the very open space feel almost claustrophobic
4) mike's bedroom is a mess
it used to be immaculate. now there are dirty clothes all over the floor and spilling out of the open wardrobe, random bits of paper everywhere (including el's letter which he crumpled up and threw on the floor), and what looks like a guitar that he's left out instead of putting away somewhere. it's overall very uncharacteristic of him
5) the changes to both of these spaces get commented on by the characters
the basement is specifically called mike's basement (as opposed to the wheelers' basement), directly linking him to the mess despite him not actually being there. you have to go more out of your way to get that effect than you do for his bedroom when he's in the same room as nancy, so it strikes me as very deliberate
tldr; when the party stopped regularly congregating at mike's house, his basement and bedroom turned into depression holes