One of my favorite MST3K movies is "Merlin's Shop of Mystical Wonders". I actually like it, even on a level above and beyond finding the riffing funny. The reason for this has a lot to do, strangely enough, with the decision to re-use footage from another movie to pad out the film.
For those of you who haven't seen the movie, it's the adventures of Merlin in the modern day; he decides to travel to the present and open up a store that sells enchanted tchotchkes in an effort to bring back belief in magic and restore the prestige of wizards. (Which actually sounds vaguely creepy, but...) The movie is clearly an old TV pilot that was repurposed as a movie after it didn't go to series, and the plot abruptly shifts halfway through to focus on a cursed toy stolen from Merlin's shop that he's trying to recover. (There's also a framing sequence that pads the film out further, which has Ernest Borgnine as a retired screenwriter telling the story to his grandkid.)
The thing is, only the first half is an old TV pilot. The second half is a heavily edited version of an existing movie, called "The Devil's Gift", about a cursed toy that slowly grows in power and threatens the life of a father and son. They cut the film down heavily, edited in some footage of Merlin looking for the toy (one of those little cymbal-playing monkeys, which does, in all fairness, look really damned creepy--every time it claps its cymbals, something dies.)
But the reason I like it is that in "The Devil's Gift", the story ends with the toy monkey just straight up killing everyone. At the end, after the hero (the dad) thinks he's gotten rid of the monkey for good, Grandma finds it and brings it home and it blows up the house with everyone inside it. Brutal, miserable, joyless, bleak, unsatisfying ending...but "Merlin's Shop of Mystical Wonders" changes it. In this version, Merlin shows up at the last second, stops the toy, and saves everyone. He takes it back to his shop, scolding it with the words, "I'll deal with you later," like he's dealing with a naughty pet.
I love this idea. I love the idea of Merlin actually going into the unhappy ending of another movie and stopping the main characters from dying. It feels like what a true hero should be doing, far moreso than the first half of the movie (the actual pilot, where Merlin delivers the comeuppance to an obnoxious reporter by luring him into dabbling with magic.) I think that they should do this with more old movies.
Hence my idea for the regular "Merlin's Shop of Mystical Wonders" series. Every week, they take an existing horror movie (like, say, "Night of the Living Dead") and recut it, adding in scenes of the Evil Toy Monkey being responsible for everything and Merlin wandering around looking for it. (Picture Barbara looking at the stuff on the mantelpiece, and then they cut to the Evil Toy Monkey.) And of course, at the end, when things were at their worst, Merlin would show up, grab the Evil Toy Monkey, and everyone would be saved! (Cut to shots of random zombies outside the farmhouse collapsing as the spell is broken.) And every week, Merlin would take the Evil Toy Monkey back to the shop, saying, "I'll deal with you later."
...or I could just be out of my ever-loving mind...