Another theme could be self - discovery. The characters might be on a journey to discover more about themselves, and their relationship helps in this process. For example, Aki might learn something new about herself through her interactions with Grey, and vice versa.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms. Just search for 'Grey x Aki' and you'll likely find a bunch of stories.
One common theme is the exploration of their relationship development. It could start from their initial meeting and gradually show how they grow closer.
Another theme is growth. As the story progresses, both Shima and Aki may grow as characters. For example, they could learn important life lessons from their interactions with each other. In some fanfictions, they overcome personal weaknesses with the help of their relationship.
Fanfiction.net is also a great place to look for masamune x aki fanfiction. There are many writers who contribute to this pairing. However, the quality may vary, so you might need to read through a few to find the really good ones. You can also check out some dedicated anime or manga fanfiction forums where fans might share their works or recommend some good ones.
One common theme is friendship evolving into love. Since they start as friends in the original story, fanfictions often explore how that friendship deepens into a romantic relationship over time.
There is a fanfiction titled 'The Journey of Shima and Aki' which is really good. It focuses on their adventures together. The story weaves in elements of their personalities well, making it an engaging read as they face various challenges side by side.
Another characteristic is the exploration of their relationship. It can show different aspects of their bond, whether it's friendship, a deeper emotional connection, or something in between. The fanfic should also be in line with the general 'feel' of the original source material, if there is any, in terms of the setting, mood, and the nature of the characters.
One popular 'Inazuma Eleven Endou x Aki' fanfiction might be 'A New Beginning for Endou and Aki'. It explores their relationship in a post - game world, with new challenges they face together.
I'm not sure if there is any really interesting 'Naruto x 50 Shades of Grey' fanfiction. Fanfiction can be very subjective. Some might find the crossover concept strange and not be interested at all. However, there could be some out there that creatively blend the two universes in a non - offensive and engaging way, but I haven't come across it myself.