I'm pretty sure I won't write this into a fic but I gotta get the plot out there
What if Daniel actually lived and Buck went through multiple donations throughout his life. Until when he was 18 he ran away because he can't take it anymore.
He's never been home after, Maggie will tell him that Daniel is ok and stable now so Buck doesn't feel like he's eating alive by guilt. Then Buck joined 118 and started his new family and has a major crazy crush which turned into hopelessly unrequited love towards Eddie.
Christopher loves Buck with no any other intentions and that makes Buck wanted to cry. He always felt people love him because they wanted something from him, for example, his parents. But Christopher and 118 proves otherwise.
Then Buck Begins is his secret revealed, Daniel came to visit him in the firehouse, Eddie wasn't impressed and he told Buck he had every right to not like Daniel because he's Bucks best friend. Buck felt funny after that.
Just when Buck thought he and Eddie might have something Eddie started dating Ana so Buck just backed out. But then one night he was half asleep or a bit drunk he called into Eddies voicemail asking him why can't you choose me, love me, I will be so thrilled to be part of you and Christopher's family, I will be a great parent to Christopher, I will be a great husband to you, etc etc. But then he dialed the wrong number, Bobby ended up listened to all his midnight cry.
Then the sniper scene, when Buck was climbing up the crane, a huge loud bang noise startled everyone and Buck thought he got shot. His hand slipped and he fell to the platform, almost broke his ankle. It was just a construction materials fell down, not sniper, but Hen was like that's it I'm going up, but Buck yelled no need, and continued to climb and got the person rescued.
After Buck landed, Chimney and Hen give him hell well tending his bruised ankle. After he got home Christopher saw his injuries and he was really really upset. Buck apologized to him and realized that Christopher has just been tough so he can protect Buck too because he knows Buck is fragile after Eddie got shot.