One popular DCUO fan fiction could be those that focus on creating new storylines for lesser - known characters. For example, a story about a new sidekick of Batman who has unique powers and has to prove themselves in Gotham. Another type might be crossovers within the DCUO universe, like a story where heroes from different factions have to team up to fight a common, powerful enemy that threatens the entire game world.
The DCUO (DC Universe Online) origin story is mainly about how your character comes into being in the DC Universe. It sets the stage for your adventures. It might involve things like your character getting powers, either through some scientific accident or a magical encounter. It's the starting point that defines who your character is and what their place will be in the world filled with superheroes and villains.
I don't think there is a Marvel game equivalent to DCUO. Usually, Marvel games have their own unique features and gameplay mechanics that set them apart.