Love and acceptance is also a big theme. Given their different backgrounds (Issei from one fictional universe and Mordred from another), they have to learn to accept each other's differences and fall in love in the process. This often involves overcomingsocial and cultural barriers within the fictional settings.
Another theme could be adventure. They might go on quests together, facing various challenges that test their relationship and individual strengths. For example, they could be on a mission to save a kingdom or defeat a powerful enemy.
One common theme is self - discovery. Both Mordred and the OC often go through a journey of understanding themselves better because of their relationship. Another theme is loyalty. They usually show great loyalty to each other in the face of various difficulties.
One common theme is the clash of their personalities. Gilgamesh is proud and haughty, while Mordred is rebellious. Their interactions often involve power struggles and attempts to outwit each other.
One common theme could be the clash or harmony of different powers. Since Issei might have his own set of powers and Amaterasu is a significant figure in mythology with her own powers, their interaction in terms of power dynamics can be a theme. For example, how they combine their powers to face a common enemy.
One common theme is the exploration of their relationship. Since they come from different backgrounds in the anime universe, fanfictions often delve into how they get to know each other better and form a connection.
Friendship is a common theme. They might show how Issei and Gasper support each other through different challenges in their fictional world.
Love and friendship are often explored as well. It could start as a friendship that gradually develops into something more. Or perhaps they are forced to work together due to some external threat and in the process, they develop deep feelings for each other. The stories might also touch on themes of loyalty and sacrifice as their relationship progresses.
One common theme is the development of their relationship. Since they often start with a certain dynamic in the original source, fanfictions like to explore how that relationship deepens and changes over time.
One common theme is redemption. Often, Mordred is shown as a character who has the potential for good despite his fated role in the Arthurian legend. Merlin is sometimes the catalyst for this redemption. For example, in many fanfics, Merlin tries to change Mordred's path towards the doom he is supposed to bring.
You can try fanfiction websites like Archive of Our Own (AO3). There are often a lot of well - written Issei x Mordred fanfictions on it. Just use the search function and type in the relevant tags.
Another common theme is the exploration of the different arcs from the series but with a focus on Issei and Rias. This could include their adventures in the Underworld, dealing with rival devil groups, or facing new threats together. Fanfictions may expand on these storylines while highlighting their relationship.