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.
One great 'Ace Sabo Luffy Fanfiction' is 'The Brothers' Bond'. It beautifully explores their relationship, showing how they support each other through various challenges. Another one is 'Sabo's Return'. It focuses on Sabo coming back into Luffy and Ace's lives and how they all readjust. There's also 'Ace and Luffy's Adventure', which takes the three on a fictional journey filled with action and heartwarming moments.
In a fanfiction, Ace's own experiences and battles could be related to Luffy's metabolism. Maybe Ace has some knowledge about a special fruit or power that could enhance Luffy's metabolism. He could share this with Luffy, leading to a new adventure where Luffy tries to obtain this thing to boost his metabolism. Also, Ace's death could have an impact on Luffy's metabolism. Luffy could go into a state of shock or extreme grief that either slows down or speeds up his metabolism in an unexpected way.
Often, the Marines can be main antagonists. They see pirates as a threat, and with Ace and Luffy being pirates, they would be constantly on the Marines' radar. Marines like Akainu are especially dangerous, as his extreme sense of justice could lead to direct confrontations with the brothers.
Well, it could be that in certain fanfictions, the writers are trying to explore 'what - if' scenarios where Ace and Sabo have some sort of falling out with Luffy. It might be because of jealousy, perhaps they think Luffy is getting more attention or something. But really, this is a huge departure from their normal loving relationship in the main One Piece story.
A prevalent theme is the exploration of their relationship in different scenarios. For example, in some fanfictions, the kisses might happen during a time of celebration after a great victory. This shows their joy and how they share their emotions with each other. Also, themes of protection often come up. A kiss can be a way for one to comfort the other when they are in danger or feeling vulnerable.
One common theme is loyalty. Their marriage in these fanfics often emphasizes their unwavering loyalty to each other. They are always there to protect one another, just like in the original story.
First, you should definitely check out AO3. It has a well - organized tagging system, so if you search for 'ace protects luffy', it will show you relevant fanfictions. FanFiction.net is also a good bet. Some of the fanfictions on there might be a bit older but still very good. Additionally, you could join some One Piece - themed Discord servers. People often share and discuss fanfictions in those communities, and you might come across some 'ace protects luffy' ones.
One popular 'ace protects luffy fanfiction' story could be 'Ace's Unwavering Shield'. In this, Ace is always there to fend off any threats towards Luffy. He uses his devil fruit powers in creative ways to keep Luffy safe during battles. Another is 'Protecting the Dreamer', where Ace not only protects Luffy physically but also defends his brother's dreams against those who try to crush them.