The theme of redemption could be explored. Maybe one of them has a past they're not proud of and the other helps them find redemption. Or the theme of cultural exchange. If they come from different backgrounds, they can learn about each other's cultures. And there's also the theme of facing challenges together. They could encounter various obstacles and overcome them as a team.
A simple yet effective title could be 'Anastasia Meets Kadoc'. It gets straight to the point. If you want to add a bit more mystery, 'The Mystery of Anastasia and Kadoc' might be a good choice. You could also consider 'Anastasia and Kadoc: A New Alliance'. This title implies that they are coming together for some purpose, whether it be friendship, a common goal, or something else entirely.
Their different personalities. Anastasia is often portrayed as having a certain elegance and mystery, while Kadoc is more complex and conflicted. This contrast can create interesting dynamics in fanfiction.
Some Fate/Grand Order fan communities on social media platforms, such as Reddit or Tumblr, might also have users sharing or discussing their own fanfictions about Anastasia and Kadoc. You can join relevant groups or follow tags related to FGO and the pairing to discover these fanfictions.
Friendship could be a major theme. Yu and Chie's relationship can start as a simple friendship and then develop into something deeper as they face various challenges in the Persona 4 world together.
Since I don't know their novels, I can't say for sure. But common themes in novels can include love, friendship, self - discovery, and social issues.
One theme could be the cycle of abuse. Harry might struggle with breaking free because he has some feelings for Tom, similar to how real - life abuse victims often have complex emotions towards their abusers. Another theme could be self - discovery. Through the abuse, Harry could learn more about his own limits and what he truly wants in a relationship.
Guilt is a possible emotional theme. For example, if Tails makes a mistake that he thinks endangers Sonic or the world, he would feel guilty and cry. Betrayal could be another theme. If someone he trusts turns against him, his crying would be a natural reaction. Also, the theme of inadequacy. If Tails feels he's not good enough to help Sonic in a crucial moment, that could bring him to tears.
One possible main theme could be friendship. Pearl and Amethyst have a complex relationship in the original series, and in this fanfiction, their relationship might be explored further in the context of sins and harmony. Maybe it could look at how their different personalities and past actions (sins) affect their ability to find harmony with each other.
One common theme is the discovery of true love. Anastasia and Dimitri often find each other in the midst of chaos and political unrest. Their love defies the odds.
One common theme is acceptance. Quasimodo's appearance has always been an issue, and in fanfiction, their relationship often shows Esmeralda's complete acceptance of him as he is.