The Duffer Brothers probably don’t even have the major beats of Season 4 plotted out yet, so the possibilities are endless.
Or, what if the Chernobyl incident is actually the doing of Upside-Down related experimentation, or a big battle in which Eleven attempts to close the Russian portal after getting back her powers?
For example, what if the disaster was a front or distraction to explain away the mysterious experiments happening in the Soviet Union?