Palpatine could be made into an ancient evil that has been dormant in the Harry Potter universe for centuries. When he awakens, he starts to spread his influence, using his political manipulation skills to gain power within the wizarding world. He could form alliances with dark wizards and creatures, much like he did with the Trade Federation and other groups in Star Wars.
In the fanfic, Palpatine could be a shape - shifter. He could disguise himself as a respected figure in the Harry Potter universe, like a Ministry official or a Hogwarts professor. While in this guise, he secretly plots to bring about the downfall of the wizarding world and establish his own rule. His true identity could be gradually revealed as the story progresses, and the heroes would have to find a way to stop him.
One idea could be a crossover where Palpatine somehow ends up in the Harry Potter universe. Maybe he tries to use the magic there to gain more power. He could attempt to manipulate the Ministry of Magic, much like he did with the Senate in Star Wars.
In a Star Wars fanfic, Spiderman's powers would need to fit into the existing lore. His web - shooting could be related to some ancient technology that was long forgotten in the Star Wars universe. It could be something that was once used by an extinct species. His spider - sense might be a result of his exposure to a strange energy field in the Star Wars galaxy. This energy field could enhance his natural abilities. And his climbing ability could be used in the many different terrains of Star Wars planets, from the sandy dunes of Tatooine to the ice - covered landscapes of Hoth.
The plot might involve Anakin being influenced by the good side of the Force more strongly. He starts having visions of a dark future if Palpatine continues to rule. Anakin then starts to secretly gather allies, including some Jedi who have survived Order 66. Together, they plan an attack on Palpatine. Anakin, with his enhanced power, confronts Palpatine directly. Their battle could be not just physical but also a battle of wills, with Anakin finally emerging victorious as he overcomes the dark side's allure within himself.
It could be Harry Potter himself. Imagine Harry being sent to the Star Wars universe. His heroic nature and powerful magic would attract various characters. Maybe Rey from Star Wars would be interested in his magic and his ability to stand up against dark forces. And some female Hogwarts students might also be part of his 'harem' as they continue to support him in his new adventures.
The story could also center around Palpatine's relationship with other key characters in the Star Wars saga. For instance, his interactions with the Jedi before he fully revealed his true nature. It could show how he managed to deceive them for so long, his schemes to undermine the Jedi Order from within. It could also touch on his influence on Anakin Skywalker's fall, exploring the details of how he lured Anakin to the dark side step by step.
It would have a huge impact on the characters. For example, Luke and Leia would grow up in a very different environment. Without Palpatine's influence, the Rebel Alliance might not have had as strong a cause, but there would still be a need to restructure the galaxy. Anakin's actions would also change the perception of the Force. The balance between the light and the dark side would shift, and the Jedi might have to reevaluate their teachings. It could lead to a more harmonious coexistence of the different Force - using groups in the long run.
One great Palpatine fanfic is 'The Dark Lord's Ascent'. It delves deep into Palpatine's early years and his rise to power. It shows his manipulations and how he planned each step meticulously. Another is 'Palpatine: The Hidden Years' which focuses on the time between his disappearances in the official lore. It's full of intrigue and political scheming.
It would likely have led to a very different outcome for the Star Wars universe. Maybe peace would have been restored earlier.
Maybe some of the Jedi Council members. They were the ones who were constantly in conflict with Palpatine's plans, so they could be in the story to show the power struggle between the Jedi and Palpatine.
They could smuggle magical creatures from the Harry Potter world into the Star Wars universe. Imagine a Hippogriff being sold on some seedy planet in the Star Wars galaxy.