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.
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.
It could be that they are trapped in a dark dimension. Here, the laws of the normal world don't apply. Ace tries to create fire, but it turns into ice instead. Marco's wings start to freeze when he tries to fly. They are surrounded by strange creatures that seem to be made of pure darkness. They have to work together to find a portal that will take them back to their world. Ace uses his intuition and Marco uses his knowledge of the One Piece world to search for clues on how to escape.
A common theme is friendship. Since Marco is an important character in One Piece, in fanfiction with a female character, they often form a strong bond. They might start as strangers and gradually become comrades who trust each other implicitly.
One common theme is friendship. Given Luffy's nature of making strong bonds with people, fanfictions often explore how he and Marco could form a deep friendship. Another theme could be adventure. They are both from the 'One Piece' world which is full of adventures, so fanfictions might create new and exciting adventures for them to go on together.
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.
It could be the Blackbeard Pirates. Since they have a history with the Whitebeard Pirates, they might capture Marco to get information or as a form of revenge.
The appeal of 'One Piece Marco x Ace fanfic' lies in the strong bond between the two characters. Their friendship in the original series is quite interesting, and fanfics can expand on it further.
One common element could be the exploration of the relationship between Ace and Marco in a more intense or troubled context, which the 'nightmare' aspect might bring. Maybe it shows them facing some sort of shared trauma or difficult situation that is related to their past or future in the One Piece world.