Adventure is also a big theme. Grey and Ash usually go on some sort of journey together, facing various obstacles and enemies along the way. This gives the story a lot of action and excitement. Another theme is growth. We see the characters develop and learn from their experiences in the fanfics.
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.
There are quite a few good ones out there. You can start by looking on popular fanfic platforms like Archive of Our Own. Some fanfics might focus on unique storylines involving the relationships between the OC, Jean Grey, and Emma Frost. For example, a story could be about an OC who has special powers and gets caught in the middle of the complex relationship between Jean and Emma, leading to a series of adventures and self - discovery.
I really liked 'A New Perspective'. In this fanfic, the OC brings a fresh outlook to the X - Men universe, especially in relation to Jean Grey. It has some really interesting plot twists and character developments. The way the author portrays the connection between the OC and Jean is both engaging and thought - provoking.
One possible plot could be that the OC is a new mutant who arrives at the X - Men school. Jean Grey senses a powerful and unique energy within the OC. Scott is initially cautious but as they all train together, the OC starts to develop feelings for both Jean and Scott. Meanwhile, a common enemy emerges that forces them to work closely, and during this time, a complex love triangle forms as Jean and Scott also find themselves drawn to the OC in different ways.
Mr. Sinister could be a great antagonist. He might be interested in the OC's unique genetic makeup, which could be related to Jean and Scott's powers in some way. He could try to manipulate the situation to his advantage, perhaps by pitting Jean and Scott against each other over the OC, or by trying to turn the OC against them.
There are indeed some interesting Grey x Erza fanfics. A lot of them delve into the unspoken chemistry between the two characters. In some fanfics, they might start off as just friends but gradually develop deeper feelings as they rely on each other more during tough times in the Fairy Tail guild.
A common theme is the clash of two different worlds. Naruto's world of ninjas and the grey lady's world, whatever it may be in the fic, often collide. Another theme could be the power of friendship. Despite their differences, they form a bond.
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.