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.
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.
Well, perhaps in the fanfiction, the reunion is not as smooth as expected. There could be some tension among the characters due to past differences. Sonia might be angry at Sonic for leaving them years ago, and Manic could be caught in the middle. They then have to work through these issues while also facing a new threat that has emerged in their absence.
A possible plot could be that Sonic from Sonic X and Sonic from Sonic Underground meet and initially have a bit of a rivalry over who is the faster or better Sonic. Then they realize they need to work together to stop an evil force that is threatening both their worlds. Another idea is that the characters from Sonic Underground are lost in the world of Sonic X and they have to find their way back home while dealing with the new threats and friends they encounter there.
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.
One interesting Sonic X Underground fanfiction could be about an alternate universe where the characters have different powers. For example, Sonic might have the ability to control time briefly in addition to his speed. Another could focus on the relationships between the characters, like a deeper exploration of the bond between Sonic and his siblings in the Underground setting. And there might be a fanfiction that combines elements from different Sonic series, creating a whole new adventure for Sonic X Underground characters.
One interesting plot could be Sonic and Maria teaming up to save a parallel universe. They use Sonic's speed and Maria's intelligence to overcome various obstacles.
One idea could be that Sonic from Sonic X accidentally travels to the Sonic Boom universe and has to adapt to the different environment and new versions of his friends. Maybe he teams up with Sonic Boom Sonic to defeat a common enemy that has found a way to cross between the two universes.
One idea could be that Sonic Underground's siblings get separated in a new zone and Shadow accidentally stumbles upon them. He initially thinks they are enemies but then realizes they are on the same side against a common foe. Another could be that Shadow is on a mission related to the Chaos Emeralds and crosses paths with Sonic Underground. They end up having to team up to stop a new villain who wants to use the power of music and the Chaos Emeralds for evil.