What makes a prequel a prequel? Damon Lindelof, in explaining why he doesn't consider Prometheus a prequel to the original Alien, comes up with a pretty good definition of the term… and one that may convince you that no-one ever needs to make another prequel ever again.
Talking on the Nerdist Writers Podcast, Lindelof said
The thing about a prequel is, as fascinating as it may be to watch Anakin Skywalker turn to the dark side of the Force, there's an inevitability to it, so you know when you go in that the prequel can only cover A to Y, and that Z is going to be the pre-existing material, and so you can't really end it in an exciting or surprising way, you can only end it in the inevitable way. So it's really just about starting a movie with Hercule Poirot saying "It is you, the butler, who did it!" and then, for the next two hours, I'm just going to watch a series of events of how Hercule Poirot came to the revelation that the butler did it. Who wants to see that movie? That's not interesting.
Specifically addressing Prometheus, he defined that movie as
By my definition of prequel, by the "A to Y" definition, I hope it's not, because I hope that this movie can be surprising, I hope that - most importantly, the ending of this movie… the sequel to Prometheus is not Alien. The sequel to Prometheus, if it does well and people like it, would be another movie that goes off in its own direction and runs tangentially to Alien.
It's a compelling idea, that "prequels" are merely "story so far"s, as opposed to stories in and of themselves. I've written before about my dislike of this inevitability of the prequel genre, the fact that you know the ending of the story before you even begin; it feels "anti-story," if that makes sense, a push against the basic force of storytelling and audience interest to find out more. No matter how inventive the prequel, no matter how many twists, turns and "You only thought you knew the story!" reveals there are, the ultimate destination is still the same one as everyone expected when they signed up for the journey. As Lindelof says, who wants to see that movie, really? The only alternative outcome is to risk destroying the original movie by revealing something that significantly changes the intent or meaning of the original movie at some point which is… risky, to say the least.
There's also something particularly interesting about Lindelof's notion that Prometheus is a parallel movie to Alien; it's the idea that another story altogether can fill in some of the gaps from your source story, but without retreading narrative. It's universe building, I guess; the kind of thing that we've seen before in spin-off and ancillary material, but it feels somewhat revolutionary in movies, for some reason - The idea of "separate but equal" stories being told in the same universe without there necessarily being an explicit connection or relationship feels counterintuitive even in this day and age of the Marvel Universe franchise being built so carefully by Marvel and Disney. It's something that - if Prometheus is enough of a hit that it spawns wannabes - I can see easily spreading to other movies, and other franchises (Star Wars, I think, could benefit from this kind of treatment, especially if the other movies were made by younger directors).
The Prometheus and Marvel model is one that I hope takes root in Hollywood, so much so that it eventually supplants "prequels" as we know them. Being inspired by a story to create another one in the same setting is, after all, a completely different - and far superior - way of paying tribute to that story than just telling people "this is what you missed, even though the original creators didn't think it was important enough to show you in the first place."