Metal Sonic from Sonic X could be reprogrammed by an unknown force to become the main antagonist. He could travel to Sonic Boom's world and start causing havoc there, using his advanced speed and combat abilities. He could also gather data from Sonic Boom's world to upgrade himself further. Sonic X's Sonic would follow him to Sonic Boom's world and team up with Sonic Boom's Sonic to stop him. Metal Sonic could create duplicates of himself using the resources in Sonic Boom's world, making it even more difficult for the two Sonics to defeat him.
A new character, a powerful interdimensional sorcerer, could be the main antagonist. This sorcerer discovers the existence of the Sonic X and Sonic Boom universes and decides to merge them into one chaotic realm under his control. He starts by causing natural disasters in both worlds to weaken their defenses. Sonic X's Sonic and Sonic Boom's Sonic would have to put aside their differences and work together. They would face the sorcerer's magical minions and his powerful spells. Their friends would also play important roles, like Tails trying to figure out the sorcerer's magic with his scientific knowledge, and Sticks warning them about the sorcerer's sneaky tactics.
Eggman Nega could be a great main antagonist. He has the intelligence and the desire for power. He could find a way to open portals between the Sonic X and Sonic Boom universes to steal resources and technology from both. His plans would force Sonic X's Sonic and Sonic Boom's Sonic to team up against him. He could use his robotic creations from both universes, combining the advanced tech from Sonic X and the more rustic, yet effective, machines from Sonic Boom.
Dark Matter could be the main antagonist in this crossover. Dark Matter is a powerful and menacing force in Kirby's world. In the crossover, it could have somehow found a way to cross over into Sonic's world and is corrupting it. Sonic and Kirby have to find a way to stop Dark Matter from spreading further. Dark Matter tries to use Sonic's speed against him by creating dangerous obstacles in his path and Kirby has to use his various powers to help Sonic overcome these while also directly fighting Dark Matter.
One possible plot could be that an ancient evil threatens both the Sonic X universe and the Sonic Boom universe simultaneously. Sonic from Sonic X and Sonic from Sonic Boom have to team up, along with their respective friends. They could start by accidentally crossing into each other's worlds through some sort of interdimensional rift. At first, they might be a bit confused and even a little competitive, but as they realize the scale of the threat, they combine their unique skills. Sonic X's Sonic is more used to high - tech environments and has different power - ups, while Sonic Boom's Sonic is more in tune with his natural island surroundings. Together, they could battle the evil and in the process, learn a lot from each other and their friends, ultimately saving both universes.
A brand - new character could also work well. Imagine a powerful sorcerer from a different dimension. This sorcerer has the ability to manipulate time and space, which would put both Shadow and Silver at a disadvantage. He could be after Sonic's chaos emeralds to increase his own power and spread his dark magic across all the Sonic universes.
I think the interaction between Sonic X's Knuckles and Sonic Boom's Knuckles would be really interesting. They both have their own sense of duty and pride, but their approaches might be different. Sonic X Knuckles is more traditional in his guarding of the Master Emerald, while Sonic Boom Knuckles might be a bit more laid - back. It would be fun to see them clash and then come to an understanding.
A new character could be introduced as the main antagonist. For example, a being from another dimension who has been observing both Sonic's and Megaman's worlds. This being realizes that by combining the chaos energy from Sonic's world and the advanced robotics technology from Megaman's world, it can create a new, all - powerful empire. So it starts to manipulate events in both worlds to bring about this combination, while Sonic and Megaman have to stop it.
Dr. Eggman and Dr. Wily could team up. Dr. Eggman has his mechanical creations and knowledge of Sonic's world, while Dr. Wily is an expert in creating powerful robots in Mega Man's world. Together, they could be a formidable force against Sonic and Mega Man.
One popular theme is adventure. Writers often create new quests for Sonic and his friends, taking them to unexplored zones. Another is friendship. They might show how the characters' relationships grow deeper through different challenges. And there's also the theme of facing new villains, with fanfiction introducing unique bad guys for Sonic to battle.
It's hard to say. Modern Sonic is very fast and experienced in his own world. But Boom Sonic has his own strengths, like his more laid - back attitude which could help him think outside the box. It could depend on the situation. If it's a race, Modern Sonic might seem more dominant. But if it's about making friends or coming up with creative solutions, Boom Sonic could shine.
Well, for me, Rouge is an interesting character to write about in Sonic X and Sonic Boom fanfiction. Her thieving ways and her double - agent - like nature add a lot of intrigue. Cream and Cheese are also cute characters that can bring a touch of innocence to the stories. You can write about how they interact with the more serious characters. And don't forget Metal Sonic. You can explore his relationship with Sonic, whether it's a rivalry or something more complex.
It could be Dr. Robotnik teaming up with the Fratellis. Dr. Robotnik might see the potential in the Goonies' treasure or the area they live in for his own evil plans. He teams up with the Fratellis who are already villains in the Goonies' world. Together, they create chaos and try to stop Sonic and the Goonies from achieving their goals, whether it's protecting the treasure or saving Sonic's friends.