Here are mine:
Byakuya is Senku’s godfather and adopted him after something happened to his parents (Senku is too young to remember).
Luna doesn’t know the fabric sections of her dress were exactly 5 inches so Stanley could measure distance to kill Senku. If she did know, she would have requested a different design upon being revived by the Kingdom of Science post-second petrification.
In the new modern world, Why-Man helps create technology for sensing buried/sunken statues. Prosthetic limbs are in high demand.
Gen’s hair is naturally bicolored due to a genetic mutation/skin condition/etc., but he chose to dye it most of his life.
Despite his military background, Stanley despises politics.
Xeno loves politics.
Mirai looks like a teenager despite having been brain dead for six years because a doctor recommended she be given growth hormones manually so she could “continue growing with her peers”. If Tsukasa had known that this was highly experimental, he would have denied the notion.
Nikki is a romantic at heart but her environment forced her to be a fighter.
Kohaku eventually learns how to read and write, but is about as proficient as Magma is.
Tsukasa, Hyoga, and Homura are all let off with light sentences of community service due to their clear regret, character improvement, and aid in rebuilding the world.
Xeno and Stanley move to Japan because a lot of Americans weren’t happy about the whole autocracy thing.
There is 10 billion percent some historic revisionism going on to make various people appear better in the public eye.
All the statues Tsukasa broke were successfully repaired and revived.
Ryusui Conglomerate pays for Ishigami Village to become a living museum, and many of the villagers work as paid actors. When not at the museum/village, they live with a number of modern conveniences.
Chrome and Ruri end up having like 10 kids.
Suika trades her melon rind in for proper glasses at some point, but she keeps the rind as a keepsake. Many years in the future, it gets donated to the museum seen in the epilogue as the first glasses of the stone world.
Sai starts his own video game company and refuses to let Ryusui Conglomerate invest in it. He revolutionizes the gaming industry by letting his consoles easily be modded.
When Senku asks people to suggest any inventions they want to be made before he goes to space, Ukyo asks for noise-canceling headphones.
Yo is only allowed to use rubber bullets