Crazy thing/idea I randomly thought

I wonder why no studio ever did this, but think about it.
Imagine for a second that a studio decides to shot an entire sequel of a succesful movie without anyone knowing. Let's pretend it's a sequel to... let's say, Indiana Jones, just as an example. Spielberg, Ford, Connery, everyone shot an entire movie after Last Crusade. And they just save it. And, boom, 40 years later they are like "So, this is a 40th Anniversary surprise, here's an unreleased surprise sequel, enjoy motherf**kers". Wouldn't that be absolutely amazing? It's a big bet for any studio (you don't see $$$ until much later, probably even more profitable), but I wonder if it could actually happen with something they know is a big hit. Like, maybe they're shooting an Avengers 4.5 right now and they release it in 2045 or something.
That's... gonna make money.