One common theme is friendship evolving into something more. Since they start with a pre - existing relationship in the game's lore, fanfics often explore how that relationship can deepen into a romantic one.
One great 'noct x iris fanfic' is 'The Bond Beyond Worlds'. It beautifully portrays their relationship with a lot of emotional depth. The author does an amazing job of creating scenarios where Noct and Iris interact in a way that feels both true to their characters and also very engaging.
There are classics like 'Bugs Bunny's Easter Special' and 'The Easter Bunny Is Comin' to Town'. They were popular for their charming characters and fun storylines.
The main storylines often revolve around their battles and conflicts for power and control of the underwater realm. There's also a focus on their personal growth and evolving relationships.