He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
Maybe he marries a character from another guild. For example, a kind - hearted healer from a guild that has an alliance with Natsu's guild. Their relationship could start off as a mutual respect for each other's skills and then develop into love.
It could be a jealous rival from another guild. They might see Natsu as a threat and figure framing him would be a good way to get him out of the way.
In a 'natsu gets married fanfiction', perhaps Natsu meets a new character who he falls deeply in love with at first sight. This new person might be from a different land. Natsu has to learn about her culture and traditions in order to marry her. His friends from the guild are initially skeptical but then come to support him wholeheartedly as they see how much he cares for this person.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
It could be a young, inexperienced mage who has always admired Natsu from afar. Maybe someone with a hidden power that Natsu senses and decides to take under his wing.
It could be a dark wizard who has a vendetta against Fairy Tail. Maybe someone who Natsu defeated in the past and is now seeking revenge.
Happy could be an interesting character. He could be like a sidekick and a source of comic relief during their relationship journey. For example, he might accidentally interrupt their romantic moments or give Natsu really bad advice on how to court the OC at the beginning. Erza could also be important. She could be the one who gives the OC advice on how to handle Natsu's wild nature, as she has known him for a long time.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
One possible reason could be that Erza might have taken on a dangerous mission alone without informing Natsu. Natsu is protective of his friends and would get angry if he thought Erza was putting herself in unnecessary danger.
Gray could be a supporting character. He might be a bit jealous at first but then becomes a good friend and helps Natsu out when he gets into trouble. Erza could also be there. She would be like an older sister figure, giving advice to Levy on how to handle Natsu's sometimes reckless behavior. Happy, Natsu's Exceed friend, would definitely be around, providing some comic relief and also helping them out in various situations.