It could be a powerful sorcerer from a magic - based crossover world. This sorcerer sees Ace's power as a threat to his own rule and plans to either capture Ace and use his power for his own evil deeds or simply destroy him.
In a crossover with another shonen anime universe, like the Bleach universe, it could be a powerful Hollow. This Hollow is attracted to Ace's strong spiritual energy (which could be related to his devil fruit power in some way). It wants to consume Ace to become even more powerful, and Ace has to use his combat skills and fire power, along with any help he can get from the Soul Reapers he meets in this new world, to defeat it.
Maybe an evil organization from a modern - day crossover setting. They might want to experiment on Ace to figure out the secrets of his devil fruit power and create super - soldiers. They would stop at nothing to get their hands on him, sending out their best agents and high - tech weaponry.
Another option for the main antagonist could be a corrupted version of one of Whitebeard's own crew members. This character could have been influenced by some dark power and now wants to take over Whitebeard's position. They would know all of Whitebeard's weaknesses and strategies, making them a very formidable foe for the crew.
There could also be a new pirate crew led by a captain who has a grudge against both Natsu and the Straw Hat crew. This captain might have a Devil Fruit power that gives him an edge in combat against Natsu's fire and the Straw Hats' unique abilities, and his goal could be to take over the Grand Line and eliminate any potential threats, which includes Natsu and the Straw Hats.
It could be a powerful pirate who has a grudge against Ace and Sabo's crew. This pirate might have some past history with them, like they were defeated by Ace and Sabo's crew in the past and now wants revenge.
One idea could be Ace crossing over into a modern - day world where he has to adapt to new technologies while still having his devil fruit powers. He might get involved in some sort of street racing using his fire powers to boost his vehicle.
Ace crossover fanfiction is fan - made stories that combine two or more different fictional universes and often involve asexual (ace) characters. For example, it could be a story that crosses over characters from 'Star Trek' and 'Doctor Who' where some of the characters are ace. It allows fans to explore new storylines and character interactions that wouldn't happen in the original works.
A powerful magical creature that has a grudge against both Harry Potter and Ace. It might see Ace as an intruder in its territory in the Harry Potter universe and Harry as someone who is interfering. So it tries to stop Harry from saving Ace and becomes the main antagonist.
If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.
A possible main antagonist could be Aizen from 'Bleach' who somehow gains the ability to control the powers from Lucy's world as well. He could use this newfound power to try and take over both the Bleach universe and Lucy's world. His cunning and powerful abilities would make him a formidable foe for Lucy and the Bleach characters to face.
Madara Uchiha could still be a great antagonist. Even with Itachi and Naruto teaming up, Madara's power and his long - term plans for the ninja world would pose a huge threat. His knowledge of the Sharingan and his connection to the Uchiha clan would make for an interesting conflict against Itachi, while his overall strength would be a challenge for Naruto as well.
Ra's al Ghul could also be a main antagonist. He might see Deadpool as a wildcard that could disrupt his plans for Gotham and the world. Ra's al Ghul's long - term goals of achieving immortality and controlling the world would put him at odds with both Batman and Deadpool. Deadpool's immortality might be something Ra's al Ghul wants to study or use for his own purposes, and Batman would do anything to stop Ra's al Ghul's evil schemes.