That monster that they tried to kill is still alive. I think, and this is my spec, that Eleven won't be able to do it alone. She already met Eight so I'm thinking there are still others out there and they might need to round up everyone to completely kill this thing. We don't need to meet them all but to truly save everyone, they're all needed to kill the monster.
Oh and of course there'd be more Steve & Dustin, I'm sure.
