@MappiTime a spin-off movie is a movie that’s usually based around a single character/subject and they’re not usually canon. Also the poster worded it wrong I think they meant; like if you think all character SHOULD have a spin-off movie. They forgot should.
A spin-off / sequel should be made years later with the same actors. It could be them all coming together again as adults to solve one last mystery revolving around the upside down. Sort of an IT Chapter 2 type theme. I actually made an Idea here: