In a fanfiction, the aliens could capture Naruto and try to study his unique abilities. The monsters then decide to rescue him because they realize his powers could be the key to defeating the aliens once and for all. The story could involve a lot of action as the monsters infiltrate the alien base to save Naruto.
In a 'Girl vs Monster' fanfic, the girl could be from a long line of monster hunters. She is the only one left in her family though. The monster she's up against is the one that killed her family. This adds an emotional depth to the story as she not only fights for survival but also for revenge. She might use her family's secret techniques to take on the monster.
One idea could be a story where the monsters and aliens have to team up against an even greater threat from another dimension. Maybe a powerful interdimensional being that wants to conquer both Earth and the alien planets.
The Missing Link is also quite popular. His brash and confident personality can add a lot of humor and action to a fanfic. He could be on a quest for glory or trying to prove himself to the other monsters.
There are many novels related to Plants vs Zombies! For example,"The Legend of the Botanic Card Creator","The Botanic Mountain Monarch","The Great Botanic Overlord","Overlord: I Summon Plants vs. Zombies in the Abyss","Where Are the Magical Plants?" Doomsday Plant Lord, Super Plant Zombie System, Family Cultivation: Starting with Plants vs Zombies, I Plant Zombies in the Other World, Playing Plants vs Zombies in the Full-time Mage World, I Cultivate in the Movies, Master Plants, Planting Herbs in the Underworld, and so on. I hope you like this fairy's recommendation. Muah ~😗
The relationship might start with curiosity. The OC could be curious about Susan's abilities and Susan could be interested in the OC's origin. As they start to communicate and work together on small tasks, like helping the monsters train for a potential threat, their relationship gradually deepens.