It’s often said that A New Hope (originally just “Star Wars”) was written as a stand-alone movie, because the good guys win at the end and the main conflict is resolved. George Lucas didn’t really know at the time if he’d ever get to make any sequels, so he wrote A New Hope to stand on its own, with all plot threads resolved by the end. The earliest known drafts of Star Wars, which are significantly different from the 1977 film, still follow the same basic story structure and end with all major plot threads resolved.
Of course, the fact that Lucas commissioned Splinter of the Mind’s Eye as a template for a low-budget sequel, just in case the 1977 Star Wars film failed, indicates he was at least thinking about a sequel even before 1977. Regardless, it’s pretty obvious that A New Hope is structured as a stand-alone film. The film’s major conflict is overcome by the protagonists (the Death Star plans are secured and used to destroy the Death Star), and the protagonist learns to use the Force (a metaphor for believing in yourself) and saves the day with help from his friend.
Of course, there’s a brief shot at the end that shows us that Vader survives. But this just comes off like Lucas hedging his bets about the possibility of a sequel. It doesn’t detract from the film’s ability to stand on its own.
However, it always struck me that if A New Hope actually WAS a stand-alone film (i.e. if no other Star Wars movies existed), there would be at least one issue that would stand out as a significant writing flaw:
The issue is that Luke’s lightsaber would become a major violation of the “Chekhov’s gun” principle. Luke is given this incredible weapon: his father’s lightsaber. Ben Kenobi even explains the significance of the lightsaber, what it meant in past times, and how Luke’s father wanted Luke to have it one day. Luke is mesmerized by it (who wouldn’t be?) Later on, Ben trains Luke a bit on how to use it. But after that… Luke never uses it again. Throughout the entire film, he never once uses it to overcome any obstacles. Sure, we see Ben Kenobi use a different lightsaber to fight Vader. But Luke’s lightsaber is setup as this really important thing - and then pretty much completely forgotten about. Luke never uses it even once to advance the plot in any way. As a standalone movie, it would seem like a good idea to simply delete all references to Luke’s lightsaber, because there is never any pay-off.
Now obviously, the pay-off eventually came around in The Empire Strikes Back. But that sort of just proves the point that A New Hope doesn’t quite work 100% as a stand-alone movie. Even if Lucas had no concrete plans for any sequels, he seemed to at least have some notion that there was more to this story.