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.
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.
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.
One idea could be that Anastasia is a long - lost Cybertronian in human form. The Autobots discover her true identity while on Earth and have to protect her from the Decepticons who want to use her unique connection to both Earth and Cybertron for their evil plans.
One possible title could be 'The Adventures of Tommyinnit and Timedeo'.
One possible title could be 'Scarecrow and My OC: An Unlikely Bond'.
A simple yet effective title could be 'Percy x OC: A Fan - Created Story'. It clearly states what the fanfiction is about, that it involves Percy and an original character, and that it's a story created by a fan.
One possible title could be 'The Encounter of Lucifer and Sabrina'. It's simple and straightforward, giving a clear idea that the story is about the two characters coming together.
Some possible titles could be 'Mitsukuni and My OC: An Unlikely Bond', 'The Encounter of Mitsukuni and My OC', 'Mitsukuni's Adventure with My OC'.
How about 'The Adventures of Ratbag and Reader'? It gives a sense of an ongoing story between them.