So this is gonna sound crazy and I urge you not to take this seriously, but hear me out... so what if whatever Hopper is helping build is a special machine that can harness the powers of the upside down... and allow the Soviets to bend reality, flay people and do brainwashing crap, and all sorts of other wierd experiments, maybe even time travel or something. The point is, it would hypothetically allow the human enemy all the powers of the Mind Flayer. Okay so just imagine how freaky it would be if some old creepy Soviet scientist guy possessed Eleven and killed Jonothan (like everyone including myself expects to happen at every season finale for some reason). Oof. Maybe Hopper can get into Wills head and have him tell everyone where he is, and then Joyce and Murray go and save him. Idk, just thought of this and felt like typing it out so here you go, another dumb fan theory to make you question the mental wellness of this fan base.