Adrien and Marinette are in their mid 20′s and married, but it’s not a happy marriage. After his father had died during the final Miraculous battle - his heart had been corrupted more and more with each forceful use of his Miraculous, leading to total heart failure when they’d finally taken it from him - Adrien had tried to get back his lost childhood by going to university and living a carefree party lifestyle. Marinette had taken over Agreste designs and Adrien was upset that she didn’t have as much time for him anymore. Marinette is getting frustrated that he’s often out at night with uni friends while she sits at home waiting for him, so she just starts staying late at work, making the problem even worse. They’re on the verge of divorce when a new enemy appears and they come across their old Miraculous, previously returned to Master Fu for safe keeping, in their respective bags.
The new enemy is stronger than they’d ever encountered before, and Ladybug leaps in font of Chat Noir to save his life. As she lays in his arms bleeding out she assures him that he has so much to live for, while she lost touch with her friends, has no children, and her husband doesn’t love her anymore. Ladybug sighs out her last breath as her eyes close for the final time, and the transformation is broken. A flash of pink light reveals his own wife, Marinette.
Adrien wakes up in a cold sweat with tears running down his cheeks. He’s in his old room and he’s a teenager again. He hastily checks the Ladyblog on his phone and finds out that there was an Akuma last night. It showed people a possible future in their dreams as a warning not to take life for granted.
He’s hurried off to school by Nathalie and he finds the rest of the class discussing what they saw in their future dreams. Rose, Juleka, and Nathanael all ran a tattoo parlour together. Chloe looks shaken and wont talk about her dream, but she seems to appreciate Sabrina more and makes her father start hosting annual charity balls in the name of breast cancer research.
Marinette was awake, initially sewing but then fighting the akuma, so she didn’t have any dreams.
The rest of the fic would be Adrien coming to terms with the fact that he has to 1) tell Ladybug/Marinette about his father being Hawkmoth, 2) defeat his own father again, which will most likely result in his death, and 3) start dating Marinette again, because he misses his wife terribly, but do it right this time.
Cue a scene with him getting to hear her spiel about Emma, Hugo, and the hamsters, then comparing it to the future he’d seen where he’d told her he wasn’t suited to be a father and she’d given up on having children.
He ends up becoming a better person and finally facing all his regrets about everything with his father. And so on, until a time skip showing a happy Marinette, Adrien, and their loving family :)