In some fanfictions, there could be an ancient evil force awakened on an island. This force might see the brothers as a threat to its plans of domination. So, it would try to stop them or even turn them against each other, which would be a great challenge for their strong brotherly bond.
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.
It could be a powerful pirate crew that has a vendetta against either Luffy or Sabo. Maybe they have some history with the Straw Hat Pirates or Sabo's group.
Another storyline could revolve around a time when they got separated in a dangerous area, like a mysterious island full of ancient traps. They both have to find their way back to each other while overcoming various challenges. Luffy might make new friends who help him, and Ace might discover some secrets about their family history in the process.
Blackbeard could also be a main antagonist. He has his own ambitions for power and the One Piece. With Luffy getting stronger under Dragon's training, Blackbeard would view Luffy as a direct competitor. Their paths are bound to cross, and Blackbeard would not want Luffy to achieve his goals before he does.
Maybe there are some shadowy figures from Robin's past who resurface because of Luffy's new power. These antagonists could be related to the organization that hunted Robin before. They fear that with Luffy by her side and being so powerful, she will finally be able to expose their secrets and bring them down. So they try to stop Luffy and Robin at all costs.
The World Government could still be the main antagonists. Since Luffy is getting stronger under Mihawk's training, they would see him as an even greater threat. They might send their top agents, like the Cipher Pol units, to try and stop Luffy's progress.
There could be an ancient evil force that awakens because of Luffy's new powers. This force was once sealed away but now wants to destroy Luffy and everything he stands for, seeing him as a disruption to the balance it once knew.
In this fanfiction, the two central characters are Ace and Luffy. Ace, being the older brother, is likely to be the more protective and guiding one. Luffy, on the other hand, is full of energy and curiosity, always ready for an adventure. Their different personalities will surely lead to many interesting events in the story.
In the 'Naruto Saves Luffy and Ace Fanfiction', the main villains might include the World Government from One Piece. They have a lot of power and resources and are often against pirates. Also, there could be some rogue ninjas from Naruto's universe who are attracted by the chaos and want to take advantage of the situation. These villains would pose a great threat to Naruto's mission of saving Luffy and Ace, making the story full of tension and conflict.
The Yonko would definitely be major antagonists. Since Luffy is in the New World, the likes of Kaido or Big Mom would be natural choices. Their vast power and influence would pose a great threat to Luffy and his crew.
Obviously, Ace and Luffy's crew which includes Luffy himself, Zoro, Nami, Usopp, Sanji and others are the main characters.