The mostly stunning second season of Stranger Things has only been out in the wild for a relatively short amount of time and already the Duffers have confirmed that the third season is coming (and that a fourth will probably follow). In other words, it's already time to start getting excited about what comes next for Hawkins' own Losers Clubs/Goonies tribute act and their devil in the Upside Down.
The show may have ended on mostly happy note, with new couples forming, new haircuts debuted and the Gate to the bad world being closed, but this is not the end of the story by any means. The Mind Flayer wasn't destroyed, he was merely defeated in a single battle - the war will rage on, and he's far more likely to be motivated now to break through into the real world now he knows his enemy even more intimately.
And even beyond that big idea, there are ramifications of the way the show ends that need to be picked at and loose ends that could come back into play in the third season, which is why there's so much room for interpretation of what the ending means. All of that detective work and interpretation will also at least give you something to do while we're forced to wait for a whole bloody year for it to come back. If not longer.