One idea could be a crossover where Naruto characters gain the ability to transform into Titans. For example, Naruto could use his chakra to control the Titan form in a unique way.
One possible plot could be that the ninjas from Naruto are transported to the world of Shingeki no Kyojin. Naruto and his friends could use their ninja skills to help the Scouts fight the Titans. For example, Naruto's Shadow Clone Jutsu could be used to create distractions while the Scouts attack. Sasuke could use his Sharingan to predict the Titans' movements. And Sakura could use her superhuman strength to take on the Titans directly.
Levi Ackerman from Shingeki no Kyojin is also a very popular character to feature. His exceptional combat skills could be pitted against Naruto's ninja techniques. Maybe he could teach the Naruto characters some of his own unique combat strategies.
In some fanfics, the themes of both series are blended. 'Shingeki no Kyojin' is about survival against overwhelming odds, and 'Naruto' is about achieving dreams and facing one's past. A fanfic could explore how the characters from both series deal with each other's problems. For instance, how would the ninjas of Naruto react to the despair and hopelessness that often fills the 'Shingeki no Kyojin' world? And how would the 'Shingeki no Kyojin' characters be inspired by Naruto's never - give - up attitude?
First, make sure to find reliable platforms like Archive of Our Own. They usually have a wide range of well - categorized fanfictions. Second, read the tags carefully as they can tell you about the genre, any possible triggers, and the nature of the story. For example, if it's tagged as 'angst', expect a lot of emotional turmoil in the story.
One interesting concept could be the combination of the unique power systems from both 'Shingeki no Kyojin' and 'Naruto'. For example, how the Titans' abilities might interact with ninja jutsus. Maybe the Survey Corps could use some Naruto - style stealth techniques to approach Titans more effectively. Another concept could be the character crossovers, like Eren Jaeger learning from Naruto about the power of friendship and how it can influence one's will to fight.
Archive of Our Own (AO3) is a great choice. It has a vast collection of fanfictions, and the search and tagging system is very useful for finding exactly what you want. Another good option is FanFiction.net, which has been around for a long time and has a large number of Shingeki no Kyojin fanfictions as well.
Eren is often a popular character in these fanfictions. Since he's a central character in the original series, in gamer fanfictions, he could be the one that the gamer teams up with or tries to influence. His strong will and determination make him an interesting character to include in these alternate stories.
One popular 'Shingeki no Kyojin' fanfiction is 'The Wings of Freedom'. It explores an alternate universe where Eren has different powers and the story takes a new turn. Another is 'Beyond the Walls' which focuses on the characters' lives outside the main events of the original series.