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 way could be through a magical portal. Imagine in a fanfic, a powerful mage in the Game of Thrones world accidentally opens a portal to the Naruto universe. Dark Naruto, with his edgy and powerful nature, could be seen as a threat similar to some of the dark forces in Game of Thrones. He might form an alliance with a power - hungry lord in Westeros, thinking he can use their resources to achieve his own goals in the Naruto world. For example, if he teams up with Littlefinger, they could plot to take over both kingdoms using Dark Naruto's powers and Littlefinger's cunning.
Well, it might involve the political intrigue. The complex politics in Game of Thrones could be combined with the NERV organization's secrecy in Evangelion. Maybe the different houses in Game of Thrones are vying for control over the Evangelion units. And the pilots of Evangelion could be seen as pawns in this political game, just like many characters in Game of Thrones are used by the powerful lords.
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.
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.
One way could be through a magical or technological device. Maybe in the fanfiction, a character discovers an ancient artifact that can open portals between the Naruto universe and the Assassin's Creed Black Flag world. This device could be powered by chakra in the Naruto side and the Pieces of Eden from Assassin's Creed. Characters from both sides would then be able to cross over and interact, leading to all sorts of interesting scenarios like Naruto characters learning the ways of the Assassins or the pirates from Black Flag getting exposed to ninjutsu.
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.
One way to merge the two universes in fanfiction could be through a plot device like a dimensional rift. For example, a Sith Lord could accidentally open a rift that leads to Gotham City. This would bring the dark and brooding atmosphere of The Dark Knight into the Star Wars universe. Batman might find himself in a world of lightsabers and the Force, and his detective skills could be put to the test in a whole new way.
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 way to merge the two universes could be through a magical portal. Imagine if the White Walkers from Game of Thrones somehow found a way into the world of Black Panther. The Wakandan technology and the magic in Game of Thrones would create a really interesting dynamic. The Wakandans with their advanced weapons and the ability to harness vibranium could potentially fight against the White Walkers. Their stealth technology might give them an edge in surprise attacks. And on the other hand, the magic users in Game of Thrones could be intrigued by the vibranium and its unique properties, perhaps trying to use it for their own purposes.
The technology aspect can also be a key point. Assassin's Creed has the Animus, which allows people to relive memories. In a fanfiction, this technology could somehow end up in Gotham. Batman, being the detective and tech - savvy hero he is, could study it. He might use it to gain insights into his own past or the history of Gotham, which could be linked to the Assassin's Creed lore. And an Assassin could come to Gotham in search of the Animus, leading to their encounter with Batman.