The best Harry Potter prequel series has already been written. Warner Bros. is eager to transform Harry Potter into a greater franchise, but unfortunately the studio is struggling to accomplish this. The Fantastic Beasts films were a solid attempt, but they've largely failed, leaving the Wizarding World in limbo.
At first the Fantastic Beasts series seemed promising, potentially setting up films that would flesh out the magical world through the exploits of Eddie Redmayne's Newt Scamander. But they became focused on the setup for a grim wizarding war, and forgot the wonder and joy of discovery that always lay at the heart of the franchise. Sensing something wasn't working, Warner Bros. and J.K. Rowling pivoted until their nature as a prequel was the main selling point for the films. Marketing for Fantastic Beasts: The Secrets of Dumbledore focused more on Albus Dumbledore than on Newt Scamander, signaling that the trilogy had lost its way. Fantastic Beasts 4 increasingly looks as though it's been quietly canceled.
The Harry Potter franchise has taken considerable damage over the last few years, in part because Rowling herself has become increasingly controversial. If a prequel series is indeed the only thing that can work, then ironically the best potential approach has already been set up; one focused on James Potter and the Marauders. Rowling has even already written a prequel story, sold at a charity auction in 2008 but also published online. Further details were, of course, recounted in the main Harry Potter books themselves - which included a number of flashbacks courtesy of magical means such as the Pensieve. This could retain the sense of wonder that characterized the Harry Potter films by prominently featuring Lily Potter, Harry's Muggle-born mother, who would serve as a perfect window into the Wizarding World.
Will A Marauders Movie or TV Show Ever Happen?
There's currently nothing to indicate Rowling and Warner Bros. are working on a Marauders film - or even a high-value TV series. The latter would potentially be a smart pivot, simply because it would lend itself well to the kind of extensive world-building and character arcs the concept deserves. Such a show could begin at Hogwarts School for Witchcraft and Wizardry, follow the traditional Harry Potter approach of each season telling the story of an academic year, and potentially move on into the Wizarding War between the Order of the Phoenix and Voldemort's Death-Eaters.
It's true that large swathes of the character arc are already known. But some of the memories explored in the Pensieve suggest most of what Harry learned was filtered through the nostalgic reminiscences of those who loved his parents. It would be fascinating to get an opportunity to really spend time with Lily and James Potter, as well as their friends, and to see how Hogwarts changed over the decades under Albus Dumbledore's leadership. The teen romance between Lily and James Potter would make the show feel truly distinctive, however, given the Harry Potter films never really excelled at romance. Harry's parents were just 21 when they died, and there would be tremendous power in viewers getting to know them in a way Harry himself was never able to.