What new classics have we declared off-limits?
Children of Men (2006)
Why someone would want to remake it: The P.D. James novel that inspired the film is pretty radically different, and one could argue that there's an entirely different film that could be made using the same source material. That's one of the best-case scenarios when you're talking about a remake, some kind of fresh take that really justifies going back to the well.
Why they shouldn't: What Alfonso Cuaron did so well was find the themes and ideas in the James novel that could support a movie and he rebuilt the experience entirely. His film works as almost pure cinema, while the book is a rich, dense piece of text that digresses and folds in history and social commentary as a way of making the big points. The characters are different, in many cases built out of several characters from the book, and Clive Owen's character was a canny reworking of the Theo in the novel, an Oxford historian. The book reads as an angry and, frankly, terrified reaction to rising immigration anxiety in England, and James painted a picture of a society that has given up on believing in a future. It's a harrowing portrait of where she believed England was headed, and it lacerates every character with equal precision. If you want a great example of the difference between film and print in terms of how stories are told, this is a case study, and any further adaptation would be hoping for a second lightning strike in the same spot, unlikely to say the least.