I had a really dumb theory about "Grease" when I was a kid. I thought Sandy was pregnant at the end of the 1978 movie musical.
Come on, go with me. In "You're the One That I Want," when she sings "You better prove, that my fate is justified," I heard (and still hear) "That my BABY's justified." It made some sense, since Danny's immediate next line is "Are you sure?" Hey, maybe Rizzo wasn't the only one who thought she had a bun in the oven.
Well, I was a stupid kid who apparently had wax in my ears, because that was NOT where the movie meant to go, and even the newest baby bulge would have shown up against Olivia Newton-John's tight black leather outfit. But I wasn't the only one making up plot tangents, apparently.
A fan theory posted on Reddit three years ago has suddenly gone viral, and while it doesn't feature a baby Zuko, it does involve Sandy. You can read Redditor atomicbolt's idea in all its glory, but the short version is that Sandy really did drown in the incident Danny sings about in "Summer Nights." The whole movie is a fantasy that flashes through her mind as she lies in a coma, and the final flying car scene is her being taken off to heaven.
Although the premise is kinda macabre, you can see where it comes from. The flying-car bit was never explained, and the whole thing about how Rydell High School could afford to suddenly have a massive carnival appear on school property wasn't either.
One similar theory you might've heard of is the Ferris Bueller "Fight Club" theory, which purports that Ferris is all in friend Cameron's imagination. There's also an intriguing proposal that James Bond is not one man, but simply a code name that many secret agents have worn -- which could explain all the recastings, and help out should a non-white (Idris Elba!) or non-male (Gillian Anderson!) actor eventually take on the role. (Want more? Lose yourself in the Fan Theories Wikia, where ideas range from reasonable to raging.)
As for the RIP Sandy theory? I'd prefer her to be alive (and not pregnant!), thank you, but as we know for cinema conjecture, it goes like rama lama lama ka-dinga da-dinga dong.