Well, for an engaging Naruto OC Inventor Ninja fanfic, you need to make your OC stand out. Give them a distinct personality. If they're an inventor, they might be a bit of a mad scientist type, always tinkering and coming up with wild ideas. When it comes to the inventions, they should be both cool and useful. Picture an invention that allows the user to teleport short distances by harnessing chakra in a new way. In terms of the plot, you can have the OC face challenges that force them to use their inventing skills in creative ways. It could be a competition among ninja inventors, and your OC has to prove themselves against more experienced rivals.
Start by understanding the Naruto world's rules and themes. In a fanfic about an inventor ninja OC, the OC's inventions should fit within the framework of ninja abilities. For instance, an invention could be a device that disrupts chakra flow in opponents. Regarding the story, a good approach is to create a conflict that only the OC's inventions can solve. This could be a new type of curse mark that the OC has to counter with their technological know - how. And don't forget to give your OC a backstory. Were they inspired by a famous ninja inventor? This backstory can drive their actions in the fanfic.