Alright this isn't a One-shot but it's a prompt. It isn't that creative but it could turn into a cool story. I just don't have enough knowledge of others fandoms to write it.
Yes it's a multicrossover.

So Danny is halfghost. After a few years he realizes he is immortal. He can die, he just doesn't age. He watches as everyone around him grows old and passes on. At least he got to tell his parents who he was. They accepted him and his descicion to have Danny Fenton disappear. Or else there would be many unexplained questions.

Danny Phantom always stayed, sometimes appearing with his sister Danielle. She had enough of his DNA to keep her alive. She's basically an artificial being so she doesn't age either. Vlad died though. He only had enough ectoplasm to give him ghost powers.

After years of protecting the Earth from ghosts he's finally managed to make a deal with most of them. They'd complete their obsessions in small ways instead of going all 'conquer the world' on Danny. If they did that, he'd let them wander around (only) Amity Park freely. They also had to help him stop others ghosts from creating chaos.

This arrangement worked so well Danny decided to join his sister in traveling around the world. He'd never seen much of it and he wanted to know what he'd been protecting all that time.

Now it's up to you.

Danny meets many different heroes (for example: Batman, Superman, Miraculous Ladybug, the Flash, Spiderman, The Avengers, IDK) in their young years.

When he sees these young and potential heroes struggling with all of the responsibilities, he decides to help them. Why not. He has all the time in the world.

At first he just helps them out in subtle ways but as time goes on he shows himself and mentors them.

The heroes are forever grateful to Danny. They do wonder where he is from and sometimes they ask him. Danny doesn't see any use in keeping secrets so he just tells them he's an immortal half ghost who used to live in Amity Park, which now has a different name and had become bigger. However, it's still very closed off to prevent the world from freaking out at the different ghosts floating around as if it were as normal as all the citizens coming home from work.

Credibility between heroes may vary, but they find themselves becoming fond of the quirky, sassy, teenage looking boy who not only helps them in fighting crime but also in seeing everything from another perspective.

However, that fondness turns into sadness when the glowing boy tells them it's time he leaves. He's taught them enough and it's time they learn for themselves. They ask him if they're ever going to see him again and he just smiles sadly and says: maybe.

Danny does miss his old life sometimes, and the people from then. But he accepts this new life and the responsibilities it carries. He can keep doing what he loves.

Sometimes he gets to see the heroes years later. He grins up at the man/woman they've become and nods. "Good job."