I think people haven't explored the idea of an "everyone lives" universe that doesn't really ignore stuff like Amane's illness enough
I know we don't get much of a descriptor of what exactly he had, but we know some facts: he was born with it, the symptoms point towards the issue being in his heart and, at the time, there was no treatment. Key words are "at the time"
Obligatory disclaimer to take all my info with a grain of salt bc, in the end of the day, I'm just some uneducated 20-something yo with too much free time and access to Google, BUT I feel pretty safe saying Amane had some kind of CHD (congenital heart disease). Back in the 50's, when Amane was born, the survival rates for kids born with CHD were very low. Heart surgery was a pretty new thing, still being experimented on, and not available for someone as young as he was. In the late 60's/early 70's a series of medical advancements made operations on young kids far more reliable and the number of early deaths began to decrease dramatically, which is pretty sad from the perspective of Amane's story. If he was born only a decade or so later, he probably could have gotten the treatment he needed. If Amane wasn't dying, Tsukasa would have never had to make a wish. Without Tsukasa's wish, they could have been happy
So if we put Amane in a setting where he is born at the same time as Yashiro, him surviving long enough to meet her and the rest of the cast should be very much possible, but surviving doesn't mean that he has no issues. A bit of research lead me to many posts from people with CHD talking about issues they had growing up, from physical pains and limitations to how people around them reacted to it, and there lies the plot potentials:
How do the Yugis as a whole deal with the situation? Do the parents explain things to the kids about what exactly is wrong with Amane or kinda leave them in the dark about the specifics in a misguided attempt to save them from worry? Are they kinda overprotective over Amane as leftover fear from the scare they got from when he was a baby? If yes, how much does it bother Amane to be coddled even as a teen? What if he tries to rebel by doing stuff they tell him not to but goes too far and actually ends up hurt from it? He canonically loves PE, so a scenario where he isn't allowed to take as much part in it as he desires is interesting to me. An "even alive he's still stuck and unable to do it all" sort of thing, a world that is better but not quite picture perfect
Focusing on Tsukasa for a moment, how does he respond to the situation? Does he act completely carefree as if it is a non issue or does he kinda worry more openly about his brother? If he saw Amane running around more than he should would he not care, try to stop him or go "ooooh im gonna tell mom and you're gonna get in trouble!"? This being a world where he didnt effectively sell his soul to the devil/get trapped in red house limbo for 50+ years it's safe to say he would be a less unhinged version of himself, but I mean, it's still him. There's plenty of room for open interpretation with how he'd actually behave
I imagine Amane kinda trying to hide the fact he has any sort of health issues at all from his friends when he first meets them, not wanting to be treated any differently, thing that would likely backfire in some way. How would Yashiro and Kou deal with a situation like that? Akane would probably know thanks to his status as class representative/vice in the student council, would Amane try to make him keep quiet about it? I can picture Akane scolding him and dragging him to the nurse's office if he ever pushed himself too hard
What if we throw Amane and Mei together and make them bound over shared medical issues? Maybe she could try and teach him to draw a little to pass time when he's forced to sit out of physical activities, he could ramble to her about whatever astronomy book he's reading. I think they'd be good friends
I could probably go on, but this is long enough as it is and I feel like I got the idea across, so I'll leave it here
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