One idea could be that Sonia discovers a hidden power within the Gundam that only she can unlock. This power could be the key to saving their world from a new, powerful enemy.
A possible plot could be Sonia getting lost in a strange dimension while investigating an anomaly related to the Gundam. The Gundam then has to enter this dimension to rescue her. Along the way, they encounter strange creatures and phenomena that test their bond and their skills.
One interesting Sonia x Gundam fanfiction could be about Sonia being a pilot in a new Gundam series. She might have to face tough battles and form a special bond with her Gundam unit. In this story, the relationship between Sonia and her Gundam could be explored, like how she customizes it to fit her combat style.
In a Naruto x Gundam fanfiction, a great concept could be a fusion of ninjutsu and Gundam technology. For instance, Sasuke could use his lightning - based ninjutsu to power up a Gundam's energy systems. Or perhaps the Gundam pilots could learn ninjutsu to enhance their combat abilities outside of their mechas. Also, the concept of the tailed beasts could be related to the Gundam. Maybe the power of a tailed beast could be harnessed to create a super - powerful Gundam.
For 'rwby x gundam fanfiction', you could explore the idea of a Gundam being powered by Aura. The Gundam could have different modes based on the Aura levels of the pilot. In terms of the plot, there could be a conspiracy where a group wants to use the combination of RWBY and Gundam technology for evil purposes. The RWBY characters would then team up with some of the good - natured Gundam pilots to stop this. You could also have a situation where the RWBY characters are trained by Gundam pilots in the art of mecha combat, leading to some interesting character development as they adapt to this new form of fighting.
It could be a rogue scientist who was once part of the Gundam project. He has his own ideas about how the Gundam technology should be used and is willing to do anything to get his hands on it, even if it means going against Sonia and the Gundam's pilot.
You can try fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fan - created works, and there's a good chance you'll find Sonia x Gundam fanfiction there. Just use the search function and type in 'Sonia x Gundam' to see the available stories.
Perhaps a story where a builder discovers a hidden ability in a common Gundam model during a build. This new ability then changes the way battles are fought in the Gundam Build Fighters tournaments.
One idea could be that Geo and Sonia get trapped in a digital realm where they have to work together to find their way out. Along the way, they discover new abilities in their Starforce forms that they didn't know they had.
A cool plot might be a fusion of the power sources. The Gundam's energy systems could somehow interact with the chakra in the Naruto world. This could lead to new types of attacks and defenses. For instance, the Mobile Suits could use chakra shields. Also, the political intrigue in both universes could merge. The shinobi villages' power struggles and the politics of the Gundam Wing world could combine, with characters from both sides having to navigate this complex web of alliances and rivalries.
One idea could be the integration of the mobile suit technology from Gundam into the Mass Effect universe. For example, Shepard's team could discover a lost Gundam unit and try to figure out how to use it against the Reapers. Maybe the advanced beam weaponry of the Gundam could be a game - changer in the fight.
Imagine if the main characters from both universes swap places. Naruto finds himself in the Gundam Seed Destiny world, having to pilot a mobile suit and fight in a war he doesn't fully understand. Meanwhile, a Gundam pilot like Kira Yamato ends up in the Naruto world, having to learn about chakra and become a ninja. They both have to adapt to their new surroundings and use their unique skills to survive. This would allow for a lot of character development as they face new challenges and make new friends (or enemies) in their respective new worlds.