Emperor Palpatine has always been the ultimate Star Wars villain. He was the evil mastermind behind everything in the original trilogy and prequel trilogy films. The Rise of Skywalker even brought him back to serve as the secretive big bad of the sequel trilogy.
Despite being one of the most cunning and formidable villains of all time, Palpatine was still defeated in Return of the Jedi and The Rise of Skywalker. He outsmarted many of his enemies, yet he also made costly mistakes that led to his downfall. Had Palpatine made different choices, he could've won in the original trilogy and sequel trilogy.
Controlled The World Between Worlds
Star Wars Rebels introduced the World Between Worlds, a mystical plane within the Force that connects all time and space. When Ezra Bridger accessed the World Between Worlds, he was able to reach into the past and save Ahsoka Tano from her own death. Palpatine tried to gain control of the World Between Worlds, but Ezra ensured that Palpatine would never be able to access it again.
Palpatine would've abused his control and used it to alter any event to his desired outcome. It would've truly given him unlimited power over the past, present, and future of the entire galaxy.
Found Obi-Wan And Yoda
Obi-Wan and Yoda trained Luke Skywalker and shaped him into becoming a Jedi Knight who refused to turn to the dark side. Without their teachings and positive influence, Luke would've been much easier for Palpatine to manipulate and corrupt.
If Palpatine had found and eliminated Obi-Wan and Yoda before they began teaching Luke, he could've shaped young Skywalker into his most powerful and obedient apprentice yet.
Exterminated The Ewoks
The Rebellion would've lost without the assistance of the Ewoks. The Ewoks on the forest moon of Endor helped the Rebellion defeat the Imperial forces sent there, which allowed the second Death Star's shield generator to be destroyed, thereby disabling the Death Star's deflector shield.
If Palpatine had exterminated the Ewoks, the Rebellion would've been soundly defeated, the second Death Star would've annihilated the Rebel fleet, and the Empire would've lived on. Exterminating a species wouldn't have been out of character for the Empire, as they did something similar when they sterilized Geonosis.
Built A Less Vulnerable Second Death Star
Palpatine should've learned from the mistakes of the first Death Star. Instead, he had the Empire build a second one that had an even more vulnerable design. Once the shield generator was destroyed and the deflector shield was disabled, the Millennium Falcon and other ships from the Rebel fleet were able to fly inside this Death Star.
Crippling the reactor core from the inside was even easier than an X-Wing shooting directly into a tiny exhaust port on the first Death Star. Palpatine leaked the Death Star plans this time to draw the Rebellion into a trap, but the trap was already flawed because of this Death Star's weaknesses.
Turned Luke To The Dark Side
Palpatine's plan in Return of the Jedi hinged on turning Luke to the dark side. Luke was supposed to usurp Darth Vader and become Palpatine's most formidable apprentice yet, one would easily crush what remained of the Rebellion and help Palpatine take complete control of the galaxy.
Palpatine underestimated Luke's Jedi values and the faith he placed in his father. This was Palpatine's undoing in Return of the Jedi and resulted in his most devastating defeat yet.
Not Broadcasted His Return To The Entire Galaxy
The opening crawl in The Rise of Skywalker informed fans that Palpatine broadcasted his return to the entire galaxy. While this was meant to get Kylo Ren's attention and draw him to Exegol, it also alerted the Resistance to Palpatine's return.
The Resistance had time to form a strategy and get support from all across the galaxy, and they were ultimately successful in defeating Palpatine and his Final Order. Palpatine should've only reached out to Kylo Ren and drawn him in instead of alerting his enemies and giving them time to save themselves and the rest of the galaxy.
Used Allegiant General Pryde More Effectively
Allegiant General Pryde's true loyalties lied with Palpatine. He served Palpatine faithfully during the Galactic Civil War, and he was given command of the Final Order when Ben Solo renounced the dark side. Pryde was a capable, composed, and effective military commander.
It was wasteful of Palpatine to keep Pryde in reserve and on the sidelines during the events of The Force Awakens and The Last Jedi. The Resistance could've been defeated and the transition to the Final Order would've been smoother if Palpatine relied on Pryde more. Installing Pryde as Supreme Leader would've been a better move than trying to manipulate the erratic and conflicted Kylo Ren.
Launched The Sith Fleet Sooner
It took Palpatine and the Sith Eternal many years to construct the Sith fleet. The fleet was ready to be launched by the beginning of The Rise of Skywalker, though. Palpatine only kept them on Exegol so the Resistance would be drawn into a trap, a turn of events that would help turn Rey to the dark side and allow Palpatine to possess her body.
If Palpatine launched the Sith fleet sooner, his plan might've actually worked better. The Sith fleet could've taken the Resistance by surprise and destroyed them once and for all. While the Sith fleet annihilated planets and took control of the galaxy, the deaths of Rey's friends could stoke her inner darkness and drawn her to Exegol, where Palpatine could still possess her.
Learned About And Exploited The Dyad Sooner
Palpatine's plan changed when he realized that Rey and Ben Solo were a Dyad in the Force. Instead of turning Rey to the dark side and possessing her, he began draining Rey and Ben's combined life force so he could restore his body.
If Palpatine had known about the Dyad, he could've exploited them much sooner and restored himself entirely. There would've been no need to manipulate Kylo Ren in order to draw Rey to Exegol. Instead, Palpatine could live on his restored body through the life force he drained from the Dyad, thereby defeating the remaining Jedi and positioning himself as the unopposed Galactic Emperor.
Stopped Shooting Force Lightning
Palpatine was killed for good when Rey deflected his Force lightning as it ricocheted back and melted his face off. If Palpatine stopped shooting the Force lightning, Rey wouldn't have been able to defeat him in this way.
It became crystal clear at a certain point that the deflected lightning was destroying him, and yet he continued to shoot it anyway. This fatal choice brought about Palpatine's second--and hopefully final--defeat.