Picture this: Oikawa and Iwa-chan saying goodbye at the airport before going to opposite ends of the world, to never play another match by each other's side, leaving behind all their childhood memories they made together. "Thank you for the past three years."
__ Continuation 1__
He waited and waited for a message, maybe he was still flying, maybe he fell asleep right when he got back but the message never came. Looking on the news he saw a plane crash, and it indeed was the one his friend was on.
The light of his life was gone. Oikawa was the one that made things interesting, the one that made his life better. When he was gone it was like Iwa-chan's sky went from blue to grey, he lost the one he'd always been with, the one who he'd known since the start. Maybe if he had convinced him to stay longer it would've have happened. He didn't even get a chance to confess his feelings like he should have, maybe Oikawa would still be alive if he did.
__Continuation 2__
Alternate ending: Let's say Oikawa lived and got there safely. Playing on an international level, it wasn't the same because he didn't have his partner. He didn't have the person who stood with him all those years, who he grew up with. No matter how many games he won it'd never be the same without him by his side. Even worse, slowly Oikawa was left on read... delivered and no matter what he did he couldn't help it. He couldn't stop Iwa-chan from moving on from him and leaving him behind, he couldn't stop losing the only one he had.