It could be an evil racing mogul. He wants to take over the racing world completely and is willing to use any means necessary. He sees Spider - Man as a threat to his plans because he protects the city where the races are taking place. So he tries to frame Speed Racer and get Spider - Man out of the way at the same time.
Maybe a time - traveling villain. He comes from the future and has knowledge of both Spider - Man's powers and Speed Racer's racing skills. He wants to change the past to ensure his future dominance. He uses his advanced technology to manipulate the races and create problems for the heroes, and they have to figure out his motives and stop him.
A likely antagonist could be a tech - savvy criminal. This person could be stealing parts from Speed Racer's cars to create some sort of super - vehicle that can outrun Spider - Man. They might also use the stolen technology to disrupt the city's infrastructure, causing chaos that both heroes need to stop.
It could be about Spider - Man and Speed Racer teaming up to stop a villain who is using high - speed vehicles to commit crimes in the city. Maybe the villain has some sort of device that can make the vehicles invisible, and Spider - Man's web - slinging skills combined with Speed Racer's driving expertise are needed to catch him.
Doctor Octopus is another option. His mechanical arms and high intelligence could make for a formidable foe. He might see Deadpool and Spider - Man's partnership as an obstacle to his grand plans. He could try to outwit them and trap them in his elaborate traps using his scientific knowledge.
The Kingpin could be a great antagonist. He's always been a thorn in Spider - Man's side, and his criminal empire and resources would pose a major threat to both Spider - Man and She - Hulk. He could hire super - powered mercenaries to fight them.
One good example could be a fanfiction where Racer X reveals his true identity to Speed Racer in a moment of extreme danger. This adds a new layer of drama to their relationship.
Another aspect is the exploration of their friendship. They might have secret meetings where they share their thoughts and feelings about racing. Fanfiction writers often play with the idea of a bond that goes beyond just being competitors. They may show how they support each other in times of personal crisis, like when Speed Racer is doubting his abilities or when Racer X is dealing with his own identity issues.
It could be a group of Quincy who have decided that Ichigo and Yoruichi are a threat to their new order. They believe that Ichigo's hybrid nature and Yoruichi's knowledge and power are things that could disrupt their plans to reshape the spiritual world. So, they start a campaign to eliminate them. This group of Quincy is led by a fanatical leader who is willing to do whatever it takes to achieve their goals, even if it means using dark and forbidden Quincy techniques.
It could be a character like Orochimaru who somehow gets access to Gotham's technology and becomes even more powerful. He could be creating new types of jutsu using the advanced tech there and causing chaos in both worlds.
One common theme is the clash of different racing cultures. For example, if Speed Racer crosses over with a more futuristic racing universe, there will be a contrast between his classic racing values and the high - tech approach of the other universe.
Maybe the Vulture. He's always on the lookout for new tech to sell on the black market. When he hears about Ant Man's unique shrinking abilities, he decides to capture Ant Man and force him to reveal his secrets. Spider Man steps in to protect Ant Man, and they have to fight the Vulture together.
Kabuto could also serve as the main antagonist. He is a sly character. He might enter the race with the intention of gathering data on all the participants, especially Naruto and his friends. He could use his medical ninjutsu to enhance his own racing abilities in unfair ways, like numbing pain to push himself further or using some sort of drug - like jutsu to boost his speed.