There's a lot of One Piece's fanfic about Ace and Luffy.
But I think that Sabo is also has many interesting development.
Transmigration based story. The fist point is once he wake up after he is shot by Celestial Dragon and saved by Revolution Army.
The thing that he do is send some letter to Ace and Luffy that he's ok. He was saved by a great man and his wish about reunion once they're grow up.
In previous life, he once huge fan of Garp. and he once train by him with his brothers. So he will make him as a target starting to remember about him through the actual experience and manga's information.
Bare hand, without devil fruit. Single handed Kaido, Linlin, Newgete in his prime. Even when he become old, his fist is still strong enough to blow up a city.
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Starting Point.
First career as Solder of Revolution is North Blue. You know that place known as many gang and mafia around.
And since he go to North Blue first, it possible to recruit Trafalgar Law to his side.
Even though Law join to Revolution Army, he will create Heart Pirates like canon.
Navy has X Drake that giving a role as pirates. So it won't be strange that Revolution Army has one too.
Since he is in North Blue, the battle against Germa is inevitable, but later somehow he can take Reiju in his side.
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Three Brother's Reunion
The place will be Alabasta. Luffy's and Ace's movement mostly similar with canon. It's Sabo's conclusion by tracking it through the newspaper.
So he make up the reason to be allowed to go there. Pirates that ruled the country is already become valid reason for it.
As a Fan
As a Brother
There's nothing that greater that it.
Ending of Alabasta arc will be sparring of Ace and Sabo.
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Summit War Arc
He knows that Ace will be captured by Teach.
Once Sabo warn him about Teach. For the world, Teach is unknown.
But, that's make him more dangerous. He grow up in Whitebeard Pirates and once he become known as someone who killed the Commander of Whitebeard Pirates.
And Sabo is sure that Ace know it more than himself since in Ace's Story, Ace is questioning why Teach don't want to become commander though he is strong enough.
Once Ace's execution is announced.
He become rush to anything.
He really think to go to marineford alone.
The reason why he has to go alone, first that he never says that his brother is second commander of whitebeard pirates. The second is that as Chief of Staff, he can't use use the organization's force for his own, his pride and sense of responsibility won't accept that
Though later, some of member or army notice Sabo's strange act and then help him. But it'll be small number.
And he still contact Heart Pirates to help him. This force is count as outer force to him.
The think that never expect is that the war become more terrifying that he ever imagine. One of the reason is Red Hair Shanks is not try to stop Kaido to interfere the war.
The End Of War:
1. Death of Edward Newgate
2. Marineford is almost sink because of Clash of Emoeror and Admiral
3. Luffy and Ace is dying
4. Sabo will lost one of his eye. There's also huge mark of burn in his body
5. Marshall D. Teach is success to steal Tremor-Tremor Fruit.
6. Sengoku's pension, also he lost his leg in this war.
7. Ace's bounty become 1 billion and Sabo will be known as an equal threat for World Government