Just to be nitpicky, I would not suggest the Matrix trilogy as an example of how sequels ruin the best movies. Yes, I agree Matrix I was way-y-y-y-y-y better than II and III. But II and III were always planned as part of the entire package. In other words, Matrix I, II, and III were just a single story, split across three films… like the original Star Wars trilogy. The only screw up is that the Wachowski Bros. allowed themselves to get caught up in the dreck of their own supposed cleverness.
I think that the 80’s and 90’s set of Batman movies show where sequels can go egregiously off track. Tim Burton’s 1989 original was great, but then Batman Forever and Batman and Robin came along and made us all sick to our stomachs.
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I’d like to see a sequel to The Incredibles.