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.
One interesting plot could be Luffy's newfound overpowered abilities helping him save Robin from extremely dangerous situations in a more spectacular way than in the original story. For example, he might be able to single - handedly take on an entire fleet of powerful enemies to rescue Robin.
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.
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.
Robin is also an important member of the Straw Hat Pirates. She is an archaeologist with the ability to sprout body parts from any surface. She is very intelligent and her knowledge of history and ancient things often comes in handy during their adventures. In the fanfiction, she and Luffy are likely to have some special interactions and storylines.
Sure. There is a good amount of 'Luffy x Robin' fanfiction out there. A lot of these fictions are based on the unique bond they share in the original anime. For example, some fanfictions might develop a storyline where Robin's knowledge and Luffy's leadership work together to solve a new and mysterious threat that the Straw Hat Pirates face, which makes for a really engaging read.
One great Robin x Luffy fanfiction is 'The Bond Beyond the Crew'. It delves into how Robin and Luffy's relationship could grow deeper on a more emotional and psychological level. It shows Robin opening up more to Luffy about her past traumas and how Luffy's unwavering support and positive energy start to heal her in ways she didn't expect.
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.
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.
One common antagonist could be a power - hungry doctor in the hospital who sees Jason and Robin as threats to his own rise in the ranks. He might try to sabotage their work or spread false rumors about them. Another could be a business tycoon who wants to take over the General Hospital and will do anything to get rid of those who oppose him, like Jason and Robin. A third could be an ex - lover of either Jason or Robin who is jealous of their relationship and tries to break them up.