The Star Wars saga would have been a lot darker if Luke Skywalker chose to attend Imperial Academy. Viewers tend to forget that Luke Skywalker wasn't an instinctive rebel. In fact, when the first Star Wars film began, Luke was considering enrolling at Imperial Academy, just like his older friend Biggs. Luke only really turned against the Empire after the deaths of Owen and Beru Lars, the aunt and uncle who'd raised him.
All this naturally means viewers have wondered what would have happened if Luke Skywalker joined the Empire instead of the Rebellion, presumably leaving Tatooine sometime before the events of the first Star Wars film. The LEGO Star Wars Terrifying Tales special riffed on the idea, with a dark vision of Luke Skywalker serving under Darth Vader himself. In reality, of course, this story didn't really do justice to Luke's character, so things wouldn't have played out anything like that special.
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If Luke had joined the Imperial Academy, this version of the Star Wars timeline would have been a lot darker - but not necessarily in the way most viewers would assume. The light of the Jedi would still likely have returned, but it would have been a lot easier for the Emperor to snuff it out. Here's what would have likely happened in this twisted Star Wars timeline.
Luke Skywalker didn't consider joining the Empire for any ideological reasons. It wasn't that he'd swallowed the Imperial propaganda; in fact, Owen and Beru appear to have sheltered him from it as much as they could. Instead, Luke's interest in joining the Imperial Academy lay in a desperate desire to get offworld. Luke considered his life on Tatooine to be unutterably boring, and he dreamed of soaring among the stars as a pilot. To the young Luke, Imperial Academy was the only route to fulfilling that dream. But, though he didn't realize it, Luke's background would have worked against him. Only a small proportion of the positions in the Imperial Academy went to people from the Outer Rim.
Still, Luke could have successfully applied to a Junior Academy at any time, given they accepted cadets as young as 13. Owen continually dissuaded Luke from doing so, presumably in part because he feared the boy would draw too much attention, but it's easy to imagine a scenario where a rebellious Luke ignored Owen's advice and applied sometime in his late teenage years. He'd have initially been taken to a Junior Academy for assessment, and then - based on his aptitudes - he'd likely have been sent to Skystrike Academy. Seen in Star Wars Rebels, this branch of the Imperial Academy trained the best TIE pilots.
This does not, however, mean Luke Skywalker would have stayed with the Empire. Star Wars Rebels confirmed Rebel sympathizers monitored candidates at branches of Imperial Academy, looking for potential recruits. They seem to have been particularly interested in pilots, likely because it was obvious a Galactic Civil War was coming. Such a skilled pilot would naturally have drawn their attention, and Luke's friend Biggs Darklighter would probably have been tasked with infiltrating Skystrike Academy to recruit him. Luke was fiercely loyal to Biggs, and his old friend would probably have been able to win him over. Luke Skywalker would likely have become a rebel after all.
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At first glance, then, this Star Wars timeline doesn't look especially different from the main one. But the key difference, of course, lies in the fact Luke Skywalker was not on Tatooine. This means Leia's desperate appeal for help would have been unsuccessful because the droids R2-D2 and C-3PO would never have found their way to Obi-Wan Kenobi. The Death Star plans would have been lost, Leia would have been captured, Alderaan would have been destroyed. The Empire would have appeared to be invincible, and the Rebel Alliance would have collapsed.
Obi-Wan would have sensed the Death Star's attack on Alderaan, however. Fearing Leia dead, he'd have used the Force to seek out Luke, and he'd have taken the boy under his wing. It's entirely possible Obi-Wan would have traveled to Dagobah with the Rebel pilot, and could have trained Luke side-by-side with Yoda. Unfortunately, things wouldn't have played out quite so well for Leia; Darth Vader would have been curious about her resistance to harsh interrogation techniques, and he'd undoubtedly have realized she was strong in the Force. The Sith would have used Leia's anger and grief over Alderaan to break her, and she'd have probably wound up becoming one of Darth Vader's Inquisitors.
In this timeline, the first Death Star was a symbol of Imperial might. The only way to defeat the Empire was to destroy the Death Star, and in so doing to prove the Empire vulnerable for all its strength. Unfortunately, it's safe to assume the Empire would not have allowed the recovered Death Star plans to get into Rebel hands once again. The Rebels would never have known about that ventral hatch, meaning they would have had to take a different strategy; they'd have needed to resort to sabotage. No doubt this mission would have fallen to Obi-Wan and Luke when the time was right. They'd have surely succeeded, if only because Obi-Wan realized the Death Star was being powered by Kyber crystals and destabilized them using the Force. This victory would likely have come at a cost, however, with Obi-Wan sacrificing himself - confident he would become a Force Ghost.
The destruction of the Death Star would inevitably plunge the galaxy into civil war, with the Empire's enemies unifying once again. But this new Rebel Alliance would have seemed a much more dangerous threat to Palpatine because he'd have sensed they were championed by a powerful Jedi. Darth Vader and his Inquisitors - including Leia - would have scoured the galaxy looking for Luke Skywalker. When Darth Vader heard the Jedi's name, he'd have realized the truth, just as he did in the main Star Wars timeline. Luke, for his part, would probably have been told his family history under more controlled circumstances, making him less vulnerable to Darth Vader's appeal to join him.
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Oddly enough, it's quite possible this twisted Star Wars timeline would have ended in a similar manner to Return of the Jedi. The Emperor could have used the second Death Star as bait in a trap to draw out both the Jedi and the Rebel Alliance, and it could well have all come to a head in his throne room. There, Luke would have faced an even greater challenge, as he dueled Darth Vader and - perhaps - revealed the truth that he had a sister. Light and dark would have collided over the Forest Moon of Endor, but it would be impossible to say how things would have played out.
This is especially true given the ground offensive on Endor, to destroy the shield generator, would have taken place with a completely different Rebel team. Without C-3PO, the Ewoks would never have joined in the battle against the Empire, and it is entirely possible the shield generator would never have fallen. In this Star Wars timeline, the fate of the entire galaxy really would have rested on the outcome of that throne room battle. Just as in the real Star Wars films, everything would have depended on a split-second's decision, with Luke standing as the champion of the light, potentially redeeming not just Darth Vader but his own sister.