The thing is, Steve and Nancy have run out of plot, but they've actually had fairly weighty roles this season. As opposed to say Jonathan, who has just sort of sat in the background doing his stoner shit with Argyle.
The Jonathan plot could end with his just being like so many teens and young adults people grew up with: they sort of fade into the background, never amounting to anything, which seems to be at least part of his issues, and the reason he did not want to see Nancy.
Eleven - Probably can't go until Season 5's finale due to her importance to the main narrative.
Probably, but it would be fitting if she sacrifices herself and brings a true end to the "stranger things" going on in Hawkins.
Will - Can't go without "bury your gays" being an issue.
...or, both Byers brothers die attempting to fight Vecna together for some reason...
Mike - Isn't story critical really this season, as he's largely being defined by his relationship with Eleven (and Will). His death could provide good pathos for both characters in the final season, but I feel like he would be given a weightier role this season other than just being oblivious to what's going on around him.
I would like to see the more mature side of him, as he was very bratty and disrespectful (to Hopper) last season, yet it was Hopper who had to come around and act in a more cordial manner to him.
Dustin - His arc is still being played too comedically to have a death seem anything but a WTF moment.
True about the comedic nature of the character, but would that not be especially tragic of the "funny one" ended up dying?
Lucas - I don't think the abortive attempt to make him a jock really counts as enough character focus, as he's dropped into the background. I also don't think they'd kill their only black main character.
A page or two ago, I mentioned the bad optics of killing of certain characters, and he was one of the few I thought about, but the Duffers do have a serious problem with the handling of Lucas; they only seem to have him step up in some crucial moment, but not really contributing anything to the "investigative" side (not since season 1 and somewhat in 2), leaving that to their apparent favorite character, Dustin.
The Duffers once said (at the start of season 3) that most teenage relationships rarely last, and we know Max broke up with Lucas--but that was due to her issues of survivor's guilt/the influence of Vecna, not some problem between the two, so with that said, I would like to see Lucas and Max together again, as they were always the most realistic couple / "fit" on the series (i.e., more real world back and forth, not directly connected to the fantasy elements).
Max - They've already done a fakeout here, I can't see them going down this well again.
Hopefully not, for the reasons stated in the comments about Lucas.
Out of all of them, I think Mike would be the most likely to die. Finn Wolfhard also has the most active filming schedule after Millie Bobbie Brown, so he's probably most okay with being let go a season early.
That would be a death of significance, pulling the
Stand By Me-style tragedy, only it happens during the group's time together, not in some coda / narration about post-series events.