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.
In fanfiction, it could be that the technology of Gundam Seed Destiny is introduced into the Naruto world. The ninja villages could start using Gundam - like mechas in their battles. This would completely change the power balance. The shinobi would need to adapt to this new form of warfare. Also, the pilots from Gundam Seed Destiny could find themselves in the Naruto world and learn about chakra, and they might try to combine their piloting skills with chakra - based attacks.
One interesting concept could be alternate endings. For example, a fanfiction might explore what would happen if Kira Yamato made different choices during key battles. Another concept could be character development that was not fully explored in the original. Maybe a side character gets more depth and a new backstory in the fanfiction.
One idea could be creating an OC who is a skilled mechanic in the Gundam Seed Destiny universe. This OC could be involved in secretly upgrading mobile suits, adding unique features that change the tide of battles. For example, they might develop a new type of beam weapon that has less energy consumption but more power.
I also like 'The New Order in Gundam Seed Destiny'. This one focuses on the political restructuring after the main events of the series. It delves deep into how the different factions interact and form new alliances. The story is full of intrigue and complex relationships between the characters.
First, you need to understand the core elements of both universes. For Gundam Seed, know about the different mobile suits, the political conflicts between the Earth Alliance and ZAFT. In Naruto, study the ninja techniques, the chakra system and the relationships between the villages. Then, find a unique concept to combine them. It could be a character from Naruto ending up in the Gundam Seed world or vice versa.
Well, you can start by creating a strong character concept for your OC. Consider their personality traits, like are they brave and impulsive or more cautious and calculating? Then, think about how they fit into the existing Gundam Seed Destiny universe. Do they side with ZAFT or the Earth Alliance? Once you have that figured out, start writing small scenes. Maybe a scene where your OC is introduced to their mobile suit for the first time. This will help you get a feel for the character and how they interact with the world around them.
One idea could be a crossover with another mecha anime like Macross. The characters from Gundam Seed Destiny could find themselves in the middle of a space battle in the Macross universe, having to adapt to the different technology and combat styles. For example, they might encounter the transforming Valkyrie mechs and have to figure out how to fight against them or even team up with the Macross pilots.
A third option could be 'Gundam and Ninjutsu'. This fanfiction focuses more on the technology transfer. It imagines what would happen if the technology from Gundam Seed was introduced into the Naruto world. How would the ninja adapt to things like beam weapons and mobile suit technology? It's a really cool concept that the author explores in depth.
Kira Yamato and a character from Code Geass, such as Lelouch Lamperouge. Kira is known for his strong beliefs in protecting life, and Lelouch is a master strategist with his own grand plans. They could either be at odds with each other due to their different approaches or find common ground in trying to bring peace to their respective worlds. Their meeting would surely lead to some intense debates and power struggles.