The mirror of Erised may not tell the future but it did for Harry and Ron

Ron guessed the mirror showed the future, but we know it just shows the user’s deepest desire. However, Ron saw himself as the head boy, quidditch captain and champion, house champion, and overall the greatest of the Weasley siblings. Harry saw himself surrounded by his family.

Ron does become a prefect, and if he went for his last year he could very well have been the head boy. He helped his house win the house cup and his quidditch team win the quidditch cup. He was most importantly part of the golden trio. He definitely has a case as the most important Weasley sibling, and definitely distinguished himself from his brothers.

Harry never gets his family back, but he does get to be with them during his battle at the end of GOF and on his way to through the forbidden forest in DH. Throughout the story he gets pictures and belongings of his family and learns much about them.

One could say that the mirror showed him with a loving family, and he definitely gains a loving family throughout the books. The Weasley, Hermione, Sirius, Lupin, Hagrid, and so on.

So overall, in these two cases you can argue that the mirror did show them their futures.