Sonic himself would be very interesting. Since there are two different versions of Sonic in Sonic Underground and Sonic X, it would be cool to see how they interact. They could have a friendly competition about who is faster or better at saving the day. It would also be interesting to see how Sonic from Sonic X reacts to Sonic Underground's family dynamic, as Sonic in Sonic X is more used to his friendship with Tails and Knuckles.
Sonic himself is usually really popular. Whether it's the Sonic from Sonic X or Sonic Underground, he's the main hero and often at the center of the action. People love reading about his adventures and how he deals with the challenges in the combined 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.
Classic Sonic could be the most interesting. He represents the roots of Sonic. When he meets Modern Sonic, he might be curious about all the newfangled things like Sonic's modern shoes or his new power - ups. His reactions to the modern elements would add a lot of charm to the story.
Well, a plot might involve Sonic from Sonic X getting lost in the world of Sonic Underground. He has to adapt to the different rules and characters there while also trying to find his way back home. Along the way, he forms new friendships and discovers new abilities within himself. It could be a story of self - discovery and cross - universe adventure.
One possible plot could be that the characters from Sonic Underground get transported to the world of Sonic X. They then team up with Sonic and his friends in Sonic X to fight a common enemy like Dr. Eggman who has somehow become even more powerful by combining technologies from both universes. The different Sonic characters would have to learn to work together, sharing their unique abilities and skills.
I think Sonic himself would be the most interesting. He's so different from the Pokemon world. His super speed and his attitude would really stand out. He doesn't have the typical powers of a Pokemon, like shooting fire or electricity. His way of dealing with problems, which is mostly running really fast and spinning into things, would be really unique in the Pokemon world.
Shadow the Hedgehog and Cerebella would be a fascinating match - up. Shadow's brooding and mysterious nature could be intrigued by Cerebella's confident and somewhat flamboyant fighting style. They might start off as rivals, constantly trying to outdo each other in combat, but gradually come to respect each other. Shadow could learn to be a bit more open from Cerebella, while Cerebella could learn the value of Shadow's more strategic and calculated approach to battles.
Sonic and Pikachu would be really interesting. Sonic's fast - paced and adventurous nature combined with Pikachu's cute but powerful personality could lead to some great comical and action - packed scenes. For example, Sonic could be running around really fast and Pikachu could be trying to keep up on his little feet while zapping enemies.
Sonic, of course, is a must - have. But also Knuckles from both universes. In Sonic Boom, Knuckles is a bit more of a muscle - brained character in a fun way, while the classic Knuckles is more serious about guarding the Master Emerald. When they meet, there could be some hilarious misunderstandings at first, like Sonic Boom Knuckles trying to show off his strength to the other Knuckles, but then they end up teaming up against a common threat.
A common theme is adventure. Both Sonic X and Sonic Underground are about Sonic and his friends going on exciting adventures, and this is often carried over into the fanfiction. For example, they might be on a quest to find a powerful chaos emerald or save a new world from an evil villain.