the rift is still open, and the Upside Down is growing into "our" world. We see that in the last couple of episodes, when Hopper and Joyce head through the rift. The entire "bath" room was overtaken, and so were the halls of the lower level of the complex.
Also, the big question still remains: what the hell is that dimension, and how did it get that way? DnD analogies can only get so far. The fact that it is a mirror dimension, but overtaken by whatever that necrotic darkness is, really only makes for a lot of questions. What happened to the people there? How long has it been like that? Obviously, it hadn't been like that always, or even for very long, since there are cars and libraries, and even Will's fort in the woods. In the library, Hopper and Joyce even come upon an unidentified skeleton, which would imply that there had been people there for far longer than the time frame of the first season. Also, even if the demagorgon is dead, there are almost certainly other horrors in the Upside Down -- at least other demagorgon monsters. There was a very Alien-looking egg in one shot.
There's also a lot of character arc loose ends, like what's up with Hopper and the government? After such a big and pretty damned public monster massacre, how is it that everyone was just allowed to go back to their normal lives?