They might have an epic battle to test Luffy's growth. Shanks could see how much stronger Luffy has become and be proud. Maybe Luffy will show off his new Haki abilities.
In fanfiction, when Sabo and Luffy meet again, they might have an emotional reunion. They could share stories of their adventures since they last saw each other. Sabo might show off his new skills or powers, and Luffy would be excited to see his brother again. They could also plan new adventures together, vowing to face any challenges as a team.
Luffy could change the past in several ways. Firstly, he could use his devil fruit powers in a more strategic way. Since he has the experience of future battles, he knows his opponents' weaknesses better. So, when facing the enemies who were involved in Ace's capture, he can defeat them more easily. Secondly, he could try to influence Ace's relationships. Maybe he can make Ace form stronger alliances with other pirate crews, which would make it more difficult for the Navy to target him. Also, Luffy could try to expose the Navy's secret plans earlier, which would change the power dynamics in the One Piece world and ultimately save Ace.
Well, one characteristic could be the focus on a more 'romantic' or 'intimate' relationship between Luffy and Ace, which is often not in line with the original platonic nature of their bond in the source material. However, this is a very controversial aspect as it goes against the typical portrayal of their brotherly relationship.
Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.
Well, 'Luffy and Ace: Unbreakable Ties' is a fanfiction that I really like. It focuses on their childhood memories and how those shaped their relationship as they grew up. There's also 'Ace and Luffy's Great Escape' which is full of action. It shows their cleverness in getting out of difficult situations. 'Luffy and Ace: The True Friendship' is a must - read too. It simply and effectively shows the depth of their friendship.
Luffy might be more confident and less like a little kid around Shanks. He has grown a lot, and Shanks would notice this. Their relationship could become more of an equal partnership in terms of respect.
Their relationship would likely deepen. They would have a stronger bond as brothers. They might understand each other's dreams and goals even better. Sabo would support Luffy's pursuit of the One Piece more actively, and Luffy would be more aware of Sabo's role in the revolutionary cause.
When Luffy and Sabo visit in the fanfiction, it changes their characters in different ways. Luffy becomes more introspective. He starts to think about Ace's values and tries to incorporate them into his own actions. Sabo, on the other hand, becomes more open about his emotions. He had been holding a lot in, but being in the places where Ace was, he can't help but let his feelings out. This makes him more relatable and also helps him to heal some of the emotional wounds from Ace's death.