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.
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.
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.
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.
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.
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.
On the other hand, it could potentially cause some friction. If their possessiveness leads to them being overprotective of each other, it might interfere with crew decisions. For example, if a mission requires one of them to be in a risky situation, the other might object due to their possessive feelings, which could create some internal conflict in the crew.
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 way is through shared secrets. For example, Ace could tell Marco a secret he's been holding about his past that he hasn't told anyone else. This would deepen their connection during their cuddle time as it shows a new level of trust.