There are some genres and paradigms that have a realistic chance of getting set up on a spec script, and others that often need an underlying property. That is: adapted from a novel, comic book, remake, etc.
Perfect example: Back when I hustled scripts for a living, several execs told me they were looking for a young adult fantasy adventure project; basically, Harry Potter. Lo and behold, I found one, and immediately called them. All of the conversations played along the same lines: “Great, send it over!” But then: “Wait, it’s based on a book, right?” When I said no, it’s a spec script, their interest vanished. In every case, they said, “Well, let’s not waste each other’s time.”
This was because the Harry Potter paradigm called for an adaptation of a novel, or even better a bestselling franchise. The paradigm wasn’t just kids/magic, it was kids/magic/based on a book. In this project’s case we still had a happy ending, because the writer of the script busted out a novel, and we were able to set up the novel with a publisher. Suddenly, the same execs who had declined to look at the script were calling to see the novel, and we were able to set up that project.
More broadly speaking: there are two main genres/paradigms that are best buoyed by underlying property, and those are fantasy/sci-fi projects, and dramas.
Sci-fi is perhaps a little easier, given that there are more comparables of bigger projects that aren’t adapted from IP (Star Wars, Avatar, The Matrix, etc.) But these are exceptions to the rule, in each case driven by the track records of established filmmakers. And there is a place for a sci-fi spec, primarily if it’s high-concept.
For example, if a story is only about the battles among the people living on Planet Spacefight on the other side of the galaxy, then we’re probably going to need an underlying property. But if the rule of those people has to go into hiding on Earth by posing as an actor on the set of Star Trek, then yeah, we can maybe swing a spec.
The same goes for Lord of the Rings-style fantasy projects. If the main story is set in Fantasylandia and it’s about the people fighting the Dark Lord of Whatever, then we need a book. But if we wed those same sensibilities to an otherwise incongruous paradigm (wacky sitcom, gritty cop drama, etc.) then we might get traction on spec, because the high-concept is in the marriage of paradigms, and not in the Fantasylandia stuff.
On the other end of the budget spectrum we find dramas. A drama feature spec has a decent shot at traction if it already has name talent attached. Otherwise, we’re very likely looking at an indie project done for a reasonable budget, e.g. a movie shot on a phone. Dramas can and do get made, obviously, but it’s way easier to get traction if it’s adapted from a novel, play, true story, etc.