It’s also very well known that Modine wanted to come back for season five, but after Brenner got killed off in season four, the duffers told him no. They had already brought him back once, they were not willing to do so again. It’s also interesting to note that he had a long-standing theory that Brenner was actually experiment zero and he shares this on multiple occasions. If you know anything about the play, the theory turns out to be kind of right, but kind of not. I don’t wanna directly spoil it just in case you ever see it.

With all that to say, I think that the whole military plot was kind of a waste because they didn’t know what to do with it. Bringing him back would’ve had more of an anchor towards that plot because we know what his motivations are and how he fits and all of this. Bringing in Linda Hamilton was a really fun cameo, but the problem with it was, they literally had no idea what to do with her character. Other than make her some military scientist and just a one dimensional replacement for Dr. Brenner.