One common theme is the clash of different worlds. For example, when Marcus Fenix crosses over into a different game or movie universe, there's always the struggle to adapt to new rules, cultures, and technologies. This can lead to some really interesting scenarios where he has to figure out how to use his skills in a completely new context.
One popular Marcus Fenix crossover fanfiction might be the one where he crosses over with the Halo universe. In it, Marcus brings his unique combat skills and leadership to new and unexpected situations within the Halo setting.
Love and romance are also quite common themes. The OC might be someone who catches Marcus's eye and they develop a romantic relationship over time. This often involves a lot of character development for both as they navigate their feelings in a world full of chaos. Additionally, the theme of redemption can be present. Maybe the OC has a troubled past and Marcus helps them find a new path, or vice versa.
There could be a fanfiction where Marcus Fenix travels to Forks. He discovers the world of vampires and werewolves. Bella Swan is intrigued by his combat skills and they form an unlikely friendship. In this story, they might exchange their different survival skills, with Marcus teaching Bella about combat and Bella showing Marcus how to navigate the complex social hierarchies in Forks.
To write a good fanfiction, focus on character development. Show how Bella and Marcus would change as they interact with each other. Maybe Bella becomes more assertive under Marcus' influence, and Marcus becomes more in touch with his emotions because of Bella. Also, use vivid descriptions. Describe the settings, whether it's the dark and dangerous world of Marcus or the misty Forks of Bella's world. This will draw the readers in.
One popular Marcus Fenix x OC fanfiction is 'A New Bond'. It beautifully weaves a story of an original character interacting with Marcus in the harsh world of the Gears of War setting. The OC in this story has a unique backstory that ties in well with Marcus's journey.
Marcus could be a bridge between the two worlds. Since he's from the Digimon world, he could introduce the concept of Digimon to the Naruto characters. His combat skills and the bond with his Digimon could be something that Naruto and his friends look up to.
Marcus and Dom could be two friends who had a very eventful relationship during their lives. Maybe they were soldiers together, or they grew up in the same neighborhood and had many adventures.
I'm sorry, but I'm not familiar with the specific 'Jayaraj and Fenix' story. It could be a very specific or local story, or perhaps it's something that is not widely known without more context.
Marcus Luttrell is a Navy SEAL. He is the central figure in his own story. He was part of a special operations mission and became the sole survivor of a tragic event during that mission.