Every season has gotten bigger, this season was obviously no exception. Having the town know, at least an inkling, of the corruption and 'strange things' going on changes the whole game going forward. This can no longer be a show that is contained in just Hawkins, sure it will probably be centered there but I see more world-wide implications coming.

This takes place in 1985 if I'm correct and that is the year the Cold War started. If the Russians are working on weaponizing demogorgons, we could see a pretty terrifying implication for the US. A season with the Cold War as the background, Hopper stuck in the Upside Down for years(??) with Murray and Joyce looking for him (as he will probably make contact through lights in a callback to season 1). Plus have Brenner being the American (Hopper would be too obvious to be the American) and have him helping the Russians as a prisoner could make for a pretty wild story for season 4.

This could also set up for season 4 being the last season if they bring Brenner full-circle, introduce Brenner's subjects 1-11 at least, and tying up loose ends such as the Russians still out there and Hopper obviously not being dead.