In this season,
"The evil force from The Swamps of Kuzatan" could be either the Upside Down or the Russians, since they're both interconnected. Like with Will's unfinished campaign, while they quell the threat for the Khuisar (Hawkins) the characters this season aren't tied into following up on the evil force itself because of how the campaign is ended.
"But no, wait. That's not thunder. It's a horde of Ju-ju Zombies!" It's worth noting that the
Ju-ju Zombies in AD&D are more intelligent than regular Zombies, stronger, and tied to the negative energy plane in the same way that The Flayed are depicted this season. The thundering referencing to them combined in the amaglam creature that formed the Mind Flayer.
Sir Mike gets bitten by the zombies while attempting to attack them (the same thing that happens to Eleven).
Mike's suggestion,
"I'll use my torch to set fire to the chambers, sacrificing ourselves, killing the Ju-jus, and saving the Khuisar. We all live on as heroes in the memories of the Kalamar." matches the ending with Hopper (which is good because in dying a hero now, he avoided becoming genuinely problematic), but this also touches on the fact that this is the end of them playing, but the campaign's story itself isn't nicely wrapped up.
This brings me to the end of the season:
Will passes his D&D books on, and Lucas' sister gets them. If anything, I think that her introduction is largely to help pass things on to a different type of storytelling. Even the way she was introduced because Dustin's lack of collarbones would've been how he got into a ventilation system before – he's grown up too. Dustin has his girlfriend in Salt Lake City (extra hilarious for me given that I just moved away from there), and with the Byers' living farther away, I expect that long-distance communication is going to be a big deal.
On top of that, the series wrapped up and tied the Satanic Panic in to the events that occurred, which means that it's likely that we're looking more at it from a perspective of information suppression if there's another season, as well as locations outside of Hawkins experiencing issues.