A few I can think of, more for remakes specifically mentioned in the OP, rather than sequels. Although considering who owns the rights, it would never happen IMHO.
The Black Hole (1979) - gothic horror movie in space? A mad man who refuses to stop his obsessions and continues by killing and repurposing those whom he was entrusted with? A conflicted crew that reacts in very human ways leading to bad consequences from all? Update the androids and modify the smarmy “heroic” robots and update the graphics while (ideally) keeping the model work for the ships (yes, I know, never happen) and I’d be throwing my money at it!
Similarly:
The Black Cauldron (1985) - much of the above, treat the source a bit more seriously, turn down the added child-friendly additions (Gurgi can work, if done per the novel, and with the seriousness of the LoTR Smeagol elements, but would require a good team), and keep the moments that are out and out scary (to this day, I remember how terrifying I found Maleficent turning into a dragon as a child!) and in the modern Fantasy friendly climate, it could be wonderful.
Things that would work as either remakes or sequels:
A Boy and His Dog (1975)
Legend (1985)
Labyrinth (1986)
Spaceballs (1987) - Aside, I can’t believe no one mentioned this yet, because with all the new material out there, they’d be spoiled for parody choices, and a huge chance to use new or appropriate aged originals!
That’s all off the top of my head. I can think of others that have been mentioned upthread, and there are some that deserve a good remake or sequel but have been disqualified by already having terrible ones (Highlander, The Crow (although # 2 wasn’t horrible, but the rest…) and there are some that deserve a great, well done sequel but would never be able to live up to the original’s perfect performances/casting/gestalt (The Princess Bride) so… 
I’m sure I’ll think of more later, this is mostly just thinking of movies in my collection that I’ve previously complained about on this subject, and speaking of that…