4. The Naked Gun 4: Rhythm of Evil
Here's one that doesn't just sound awful, it sounds depressing, too. The idea of a fourth Naked Gun film had been kicking around for years, with Leslie Nielsen's Frank Drebin essentially becoming a mentor to a new cop who would presumably be only slightly less idiotic than Drebin himself. A script was even written, and reports from those lucky enough to read it suggest it was actually worth putting into theatrical production, yet it was packaged as a low-fi straight-to-DVD venture, one which eventually came to nothing due to financial complications. Not long after this, Nielsen died, completely obliterating the prospect of a sequel ever getting made, though given how the third film already signified a clear decline from the uproarious first two, perhaps it's best that this was left alone.