Well, Ben is obviously one of the main characters as the story is centered around him and the Omnitrix. Gwen might also be a main character as she often plays an important role in Ben's adventures, providing support or using her own magical abilities to help out. Grandpa Max could be another main character, offering his wisdom and experience to guide Ben through various situations.
One interesting storyline could be about Ben exploring new alien forms that were previously unknown in the official series. Maybe he discovers a form that has unique powers that he has to learn to control quickly to face a new and powerful enemy.
One idea could be that Ben discovers a new and powerful alien form in the Omnitrix that has abilities he's never seen before. He has to quickly learn to control it to defeat a new and dangerous villain.
One possibility is that the story could explore how Gwen's personality changes when she has the Omnitrix. She might become more confident or more responsible. Also, there could be new villains who are attracted to the Omnitrix now that Gwen has it. She would then have to face these threats and protect the Omnitrix from falling into the wrong hands.
There are many great ones. You could check out fanfiction.net. Some might focus on their adventures together, exploring new worlds or facing common enemies.
In this fanfic, the relationships among the Omnitrix aliens - as - humans would be full of surprises. The competitive relationships between some aliens might turn into friendly rivalries. Take Diamondhead and Four Arms, for instance. As humans, they might compete in sports rather than in battles. Their relationships with humans who don't know their true identities would also be interesting. They would have to constantly lie to maintain their cover, which could lead to feelings of guilt and isolation. But at the same time, they could form deep and meaningful relationships with those who accept them for who they are, despite their secrets. This would create a web of relationships that are both complicated and fascinating to explore.
The story could start with Gwen unexpectedly getting the Omnitrix. Then, she might start exploring the different alien forms she can transform into. She'd likely test out their powers in various situations, gradually getting a handle on how to use them effectively.
Kevin could also be a good pairing. His ability to absorb matter and his complex backstory could lead to some interesting storylines with Female Albedo. They could start off as rivals, both vying for power or resources, but then end up having to work together. Rook is another option. His by - the - book attitude could clash with Female Albedo's more unorthodox ways, creating comical and exciting situations in the fanfiction.
Vilgax could be a great main villain. He has a long - standing rivalry with Ben and his presence would add a lot of tension. He could see Albedo as a useful tool or a threat depending on the situation. Another option could be an original villain created specifically for the crossover. Maybe a powerful alien entity that wants to use Albedo's knowledge and Ben's powers for its own evil purposes. A third choice could be Aggregor, who might be after some new power source that is related to both Albedo and Ben.
One idea could be that Albedo creates a device that can swap powers with Ben, leading to a lot of chaos as they both struggle to adapt to the new powers. Another could be that Albedo and Ben are forced to team up against a new powerful enemy that threatens both their worlds. A third idea might be that Albedo gets stuck in Ben's body somehow and has to pretend to be Ben while dealing with his friends and family.