If there's still a story to tell I think a sequel is fine as long as its a good story. The first movie doesnt necessarily have to leave it open though.
But then there are others where it should've been left as a stand alone movie as some sequels actually hurt the first. Insidious 2 comes to mind. I liked it but I think the old lady was a lot scarier when we didn't know her entire backstory.
Other sequels can just ruin a good movie with oversaturization. The Saw films, Paranormal Activity, REC, and even the classics.
A sequel is tricky but I think the movie should decide if there's a possible sequel and not how much money it made.
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