Yes, I know it's insane, but if you're talking about remaking and improving bad movies, then certainly you have to talk about one that actually has attained a perverse fame as the Worst Movie Ever Made. So let's discuss Manos--the Hands of Fate.
For starters, let's recap what we laughably refer to as "the plot". A couple vacationing with their child and dog get lost (in a long, dialogue-free sequence) and winds up at a mysterious ranch home run by a weird guy named Torgo with big knees. Torgo takes care of the place "while the Master is away". Oddly, the couple sweet-talk Torgo into spending the night there, then change their mind, then have to stay there anyway because their car won't start. Then Torgo makes a pass at the wife, the kid finds a room full of sleeping women (and a sleeping guy, the Master), the girls wake up and start wrestling, Torgo gets his hand cut off, and finally the Master enslaves the wife and daughter (which is really icky, because she's about eight.) And the husband becomes the sort of New Torgo for the next batch of unlucky travelers. All this is intercut with scenes of two cops chasing necking teenagers.
So, OK, let's go back to square one, and improve the budget, production values, acting, and keep the skeleton of the plot. A couple is on vacation with their teenage daughter (we'll push her age up about ten years, to make that less weird.) They get lost, in a much shorter scene, and wind up stopping at a house in the desert to ask directions. Their passing is witnessed by two teenagers, who wonder why anyone would drive down a road that leads nowhere.
As soon as they stop the car, it dies and won't restart. They're forced to ask the caretaker for a place to stay for the night. Still Torgo, still with the lumpy and unusual body, but this time we're actually going with the production team's original intent, which is that it's lumpy because he's not human anymore. Manos, the god of primal darkness, slowly changes anyone who stays too long in this sacred place, and Torgo's body is metamorphosizing.
They bring their stuff to their rooms and Torgo insists that they have to leave in the morning, and shouldn't poke around. The parents agree (because they're sensible), the daughter doesn't (because she's rebellious.) She sneaks off to take a look around. She goes down into the basement, and finds that there's more basement...and more, and more, and more. And at the heart of all this, she finds a black altar with a sinister man lying on it, completely immobile, and several women, all also immobile, on altars of their own. The women are all in various states of transformation--all beautiful, in their own way, but many in a fashion that's clearly no longer human. The daughter's approach awakens them, and their inhuman eyes snap open...
Meanwhile, the cops come to harass the two teenagers, and the teenagers give them attitude about how they should maybe go harass that other couple that drove up the road with the teenage girl. The cops call their bluff, and tell the teens to show them where they went.
Back at the house, Torgo lures Dad away with the promise of finding them something to eat in the kitchen, then knocks him out and stuffs him in the pantry. He then goes back and starts macking on the mom, with promises that are (to her) nonsensical, promises of finding a way to escape the house together. (He realizes that with Dad here, Torgo's now redundant.) She freaks when he takes his shirt off and she sees that a) he's not human under all that, and b) he's seriously macking on her and she doesn't know where her husband or daughter is. She flees deeper into the house.
Daughter, meanwhile, is fleeing out of the house. The Master has awakened, and he's planning to add her to his collection of "brides of Manos". He's struck a deal with the dark god; he procures fresh brides, and he gets to stay immune to the corruption that affects everyone else. The god's power makes it more difficult to leave the longer you stay; after a while, you just can't bring yourself to leave, no matter how urgent it seems. Once the Master leaves, the women discuss whether it would be more merciful to just kill the new arrivals. This discussion gets violent, as one of the "eldest" brides, who is furthest along in her corruption, insists that although they can't rebel, the new brides still might have the strength.
The Master bumps into Torgo, and decides to take this opportunity to deliver his termination papers. He accelerates the corruption until Torgo's nothing more than a misshapen beast, then sends the beast to hunt the girls. They, meanwhile, have bumped into each other, and are looking for Dad. Instead, they find the cops and the teens, who are fairly surprised to see a decades-old house where there was nothing but desert a few days previously. The cops deal with the Torgo-beast, and go looking for Dad.
Dad, meanwhile, has been released from the pantry...by the Master. He's already been corrupted, just slightly, and feels compelled to obey. He sets about luring the group deeper into the house.
At the edge of the basement, though, Daughter panics. She's been there, she knows what's down there, and she's not going any further. She runs back for the cars. Mother is torn between following her daughter and finding her husband; the other teenage girl, though, makes it easy for her by offering to follow Daughter and make sure she's OK.
The remaining four descend into the basement, where the Master has re-established his hold on his brides. Dad is by his side as his new servant, and all around him, the darkness rustles with the sounds of movement. See, he has room for any number of brides in the harem, but he only needs one caretaker...
From out of the darkness, dozens of fully-corrupted brides spring forth. They drag the teenage boy into the shadows and consume him--one of the cops, though, manages to get a shot off at the Master. He falls, dead. The brides, all of them, stop their assault. They smile. Could it be that they're free?
Nope. From out of the Master's corpse, darkness boils up--Manos itself, god of primal darkness. It flows into one of the cops, entering through his mouth, nose, and eyes, and when he looks up, he's filled with dark power. He commands the brides, and they rip apart his former partner. Then he sends them after the two remaining girls.
Back upstairs, the girls are headed out...except that one of them can't seem to make herself leave. She sees Daughter crossing the threshold, running for the car, but...she just can't do it herself. Behind her, she hears the brides coming for her. Daughter encourages, pleads, even grabs her arm and yanks, but it's of no use. She's been trapped here. She hands the keys to Daughter and tells her to go and don't look back. Daughter, being no dummy, does exactly that.
The epilogue is substantially as in the original: A new set of victims drives up to the same house, and we see Dad at the steps, wearing baggy clothes that conceal his transformation, explaining that "he takes care of the place while the Master is away." In the basement, we see Mom and the teenaged girl, asleep and already some distance along in their transformation...
Of them all, I think this is the one that has the best chance of actually getting made--after all, you make remakes to capitalize on the notoriety of the original, and if there's one movie that has plenty of "notoriety", it's 'Manos'.