Perhaps in the fanfiction, the Shield has a hidden history that is related to the Prisoner of Azkaban. For example, it could have been created by an ancient wizard who was wrongly accused like Sirius Black, and its true purpose is only revealed when Harry and his friends start to dig deeper into the mystery. This could add a whole new layer of intrigue to the story.
Another plot idea might be that the Shield is actually a key to unlocking a secret chamber within Hogwarts that has a connection to the events of the Prisoner of Azkaban. The characters have to figure out how to use the Shield in a series of trials and challenges, all while dealing with the threat of Sirius Black and the Dementors. This would create a lot of suspense and adventure in the fanfiction.
One interesting plot idea could be a new character being introduced who has a special connection to the Shield in the story. Maybe this character has the power to enhance the Shield's abilities and uses it to protect Hogwarts in a different way than expected.
One idea could be exploring what would happen if Sirius Black was recaptured before Harry could prove his innocence. Another could be a story where Peter Pettigrew managed to frame someone else entirely. Or perhaps a fanfic where Hermione uses her time - turning device to go back and prevent Sirius from being sent to Azkaban in the first place.
One idea could be exploring Sirius Black's life more in depth during his time on the run. Maybe he had some secret encounters with other magical creatures that he never told Harry about.
One idea could be that Harry Potter is secretly recruited by SHIELD while still at Hogwarts. He uses his magic to help SHIELD in covert operations against dark wizards who are also involved in muggle - world threats. For example, he could use the invisibility cloak to gather intelligence.
Maybe Harry could be a double agent. Working for the wizarding world while also being part of S.H.I.E.L.D. He could be trying to keep the two worlds from colliding in a dangerous way while also using resources from both sides to fight common enemies. For instance, using S.H.I.E.L.D.'s technology to track down dark wizards who are using muggle - made weapons in their evil plans.
One idea could be exploring Sirius Black's time on the run in more detail. Maybe he had some secret encounters with other magical creatures that he didn't tell Harry about. Another could be a story from the perspective of Remus Lupin during his time as a teacher at Hogwarts that year, and how he was constantly worried about his werewolf secret being exposed while also trying to protect Harry. A third idea could be a fanfic that focuses on Hermione's time - turning device, and what would happen if she accidentally sent herself back too far in time and had to find a way to get back to the right time without disrupting the timeline too much.
One plot idea could be that Shield trains Harry in combat and espionage skills from a young age. This makes him a more physically and tactically adept wizard compared to the original story. He might use his new skills to uncover dark secrets within the wizarding world earlier.
A fun plot could be that some of the Agents of Shield are actually Squibs or have some latent magical ability that they didn't know about. When they cross paths with the wizarding world through Harry Potter and his friends, they start to discover their magical side. This could lead to a story where they are trained in magic while also using their Shield training to help the wizarding world fight against a new threat, like a group of dark wizards who are trying to create a portal to bring in more powerful dark forces from another dimension.
Well, Harry Potter would likely still be one of the main characters. He might be the one who discovers the significance of the Shield. Hermione Granger could also be a main character, using her intelligence to figure out how the Shield fits into the overall story. Ron Weasley too, as he's always by Harry's side in adventures.
A cool plot twist might be that the time - turner Hermione uses has some unforeseen side effects. Instead of just allowing her to attend more classes, it starts to create glitches in time that cause different events from the past to bleed into the present. For instance, old spells from the founders of Hogwarts start to randomly appear in the corridors, and the characters have to figure out how to stop this chaos while also dealing with the Sirius Black situation.
One common plot idea is exploring Sirius Black's life more in - depth. Maybe a fanfiction could focus on his time in Azkaban before his escape, showing his mental state and how he coped. Another idea is changing the roles. For instance, what if Hermione was the one who was wrongly accused instead of Sirius? It would create a whole new dynamic.