If Sirius Black were to look in the Mirror of Erised at the age of ten, he'd've seen his family. Not his family as they were, but as he wished them to be. Smiling and happy, no scowls or distance or anger at all. Normal and carefree, without prejudice or hatred or beatings or lectures. Without disappointment and words that made him cringe and ways that made his stomach turn in discomfort. A family that loved him, each other, a family he belonged to.
All he wanted, in the deepest darkest parts of his heart, was a family, in the truest sense of the word.
If James Potter were to look in the Mirror of Erised at the age of ten, he'd've seen himself. Not himself as he was, but as he wished he could be. Older and handsome, Quidditch World cup triumphantly in hand, cheering fans on all sides. Decked out in his uniform, the star chaser of England's National Team. His dream would be shallow and selfish but he is a ten year old boy and his dreams shouldn't need to be anything other than fanciful flights, because innocent children shouldn't need to wish for deeper things.
All he wanted, in the deepest darkest parts of his heart, was what most children dreamed of, something grand and exciting.
If Sirius Black were to look in the Mirror of Erised the night before his death, he'd see James Potter. Not James Potter as he was, but as he wished he could be. A James that was alive, a James that was happy and grinning, with an arm so casually thrown across Sirius' shoulders. He wouldn't be surprised to see James but he'd wonder if he should be ashamed, ashamed just to wish for James when there were so many things he could wish for, an end to the war, peace and happiness for his friends, for Harry.
All he wants in every fiber of his being is for James Potter to be alive. That's all.
If James Potter were to look in the Mirror of Erised the night before he died, he'd see his family. Not his family as they were, but as he hoped they would be. He would see them alive and happy. And not just Lily and Harry, but Sirius and Remus and Peter. All of them, together, war over and living their lives. He'd see it and promise to make it all real.
All he wanted in every fiber of his being was for everyone to be okay. That was all.
(Sirius Black never looks in the Mirror of Erised, and maybe that's for the best. Imagining hurt enough, actually seeing what he could never have might have been one blow too many)
(James Potter never looks in the Mirror of Erised and maybe that's okay. He doesn't need a mirror to show him what he wants most, he's always known that.
It's the getting that's the problem)