There's a fanfiction called 'Under the Same Sky'. It takes place during a time when the Whitebeard Pirates are facing a powerful enemy. The story shows how Marco and Ace rely on each other, their trust and loyalty shining through. It's full of action scenes as well as some tender moments between them.
A popular fanfic is 'Under the Same Sky: Marco x Ace'. It delves deep into their characters, exploring their individual strengths and how those qualities blend when they are together. It also has some great action scenes and emotional moments that are true to the spirit of One Piece.
I'm not sure of specific 'One Piece Ace x Marco nightmare' fanfictions off - hand. But you can try searching on popular fanfiction websites like Archive of Our Own. They usually have a large collection of One Piece - related fanfictions and you might be able to find ones with this specific theme.
One possible scenario in this fanfiction could be that Ace and Marco are facing a powerful and unknown enemy in a spooky, haunted island. The island is filled with strange noises and dark shadows. Ace's fire powers seem to be ineffective against this new threat as it keeps disappearing and reappearing. Marco, with his phoenix abilities, tries to scout from the air but gets caught in some sort of dark mist. They have to rely on each other and their friendship to overcome their fear and find a way to defeat this mysterious foe.
Growth is a prevalent theme. As they go through various battles and experiences in the fanfictions, they grow both as individuals and in their relationship. Their understanding of each other and their own abilities develops. Additionally, the theme of brotherhood is strong. They are like brothers in the crew, and this brotherly love is often a central focus in these fanfictions.
Maybe it's a cold night at sea. Ace, who's not used to the cold as much as Marco, huddles up to him. Marco, being the kind-hearted person he is, just pulls Ace closer. They start reminiscing about their adventures and friends, creating a really warm and fuzzy moment.
One of the key themes could be loyalty. In a possessive relationship in this fanfiction, Ace and Marco might show extreme loyalty to each other. They could be possessive because they don't want to lose what they have in each other. Another theme might be sacrifice. Given their personalities in One Piece, they might be willing to sacrifice a lot for the other in this possessive dynamic.
One great 'ace x reader x marco fanfiction' is 'A Triad of Hearts'. It beautifully portrays the relationship between Ace, the reader, and Marco. The story has some really touching moments where the characters interact in unique ways. Another one could be 'Among the Flames and Wings' which focuses on the adventures the three might have together. It has a good plot that keeps you engaged throughout.
One popular place to find marco x ace x thatch fanfiction is Archive of Our Own (AO3). Just go there and search for the relevant tags. You'll likely come across a variety of stories with different themes and plotlines.
One great 'One Piece Smoker x Ace fanfiction' is 'Smoker's Redemption'. It explores an alternate universe where Smoker has to team up with Ace for a mission and their relationship develops from there. Another one could be 'Ace and Smoker: An Unlikely Alliance' which delves into how their different personalities clash and then find common ground.
There's 'Sabo and Ace: The Forgotten Days'. This fanfiction focuses on their early days together. It shows how they first met, how they started to trust each other and form their brotherly bond. It also has some interesting sub - plots involving other characters from the One Piece world that add more depth to the story.