Upon being diagnosed with lung cancer, a lifetime litterbug tries to make the world beautiful for his depressed granddaughter before he dies.

Hi Scott.

Okay - let’s see…

Upon being diagnosed with lung cancer, a lifetime litterbug tries to make the world beautiful for his depressed granddaughter before he dies.

Intention: to make the world beautiful. (I think this needs to be more specific. How so? How to make the world beautiful.)
Obstacle:

When his granddaughter is diagnosed with terminal cancer, a timid old man sets out to find a cure before it’s too late.

(In the process - he shows her the world is beautiful/or can be a beautiful place filled with good people.)

I’m not sure why he was a litterbug. (Or at least it’s not necessary to know in the logline.)

Good luck with this!

Maybe depressed at the “endless litter”, so it’s more of a challenge, and aiming for making the town litter-free for a more meaningful goal.

He doesn't die THAT quickly and people do litter continuously, so just picking up all the litter once isn't much of a story, so I made it so he keeps cleaning. Also, I don't think it's that important to specify whether or not he's picking up the trash himself, which might be his initial plan but certainly isn't the result:

"Upon seeing his granddaughter saddened by litter, a terminally-ill litterbug and aims to keep their town litter-free until he dies."