A plot could be that the OC is from an enemy faction at first but then realizes the truth about Optimus Prime and his cause. The 'lemon' element might be that they start to fall for each other during a time when they are hiding from their respective sides. Their relationship could be full of secrecy and longing as they try to figure out how to be together in the midst of all the chaos.
Perhaps a plot where Bumblebee discovers a hidden power within himself with the help of Optimus Prime in a world filled with lemon - like energy sources. This power could be crucial in saving their planet or defeating a powerful enemy. The story could explore how they work together to harness this power and the sacrifices they might have to make along the way.
There could be a fanfiction where baby Optimus Prime is sent to the past, to a time when the Transformers' society was very different. He has to adapt to the old ways while still holding onto his future - self's values. He might meet young versions of his future friends and enemies, and his presence starts to change the course of history in small but significant ways.
A plot could be that Optimus Prime, in his travels, discovers a lost civilization. There, he meets a female leader who is intelligent and kind. She is intrigued by his noble nature, and he by her leadership and the unique culture of her people. As he helps her protect her civilization from various threats, their relationship blossoms into love.
One plot could be that they are on a mission in an unknown planet. They get lost and have to find their way back. During this time, they rely on each other's skills and start to understand each other on a deeper level. The lemon part could be the trust they build which is like a warm, fuzzy feeling in their relationship. For example, Bumblebee might find a way to communicate better with Optimus Prime during this tough time, and Optimus Prime in turn, protects Bumblebee more fiercely.
A plot could revolve around a young fan who writes a lemon - themed story about Optimus Prime. The story somehow comes to life and starts affecting the real Optimus Prime and his world. He has to then find a way to reverse the effects while also learning about the power of human creativity and how it can have unexpected consequences on their reality.
To write a good Optimus Prime OC Lemon Fanfiction, you need to focus on the relationship between the OC and Optimus Prime. Build it gradually. Start with small interactions and let it grow. The 'lemon' part should be an organic part of the story, not just added for shock value. Also, research the Transformers universe well so that the story fits within its lore. For instance, understanding the different factions and their goals can add depth to the story.
One possible plot could be that Optimus Prime discovers he's pregnant through some sort of strange energy transfer. He then has to deal with the reactions of his fellow Autobots. Some might be shocked, while others try to support him. Another idea is that the pregnancy is a result of an experiment by the Decepticons gone wrong, and Optimus has to protect himself and the unborn life from their attempts to use it for evil.
A good storyline might involve the OC being from a different dimension. Optimus Prime discovers this when a strange energy signature leads him to the OC. They have to work together to figure out how to send the OC back to their own dimension while also dealing with the Decepticons who want to use the OC's dimension - hopping ability for their own evil plans.
One interesting Optimus Prime fanfiction could be about his origin story in a different universe. Maybe in this story, he was created by a group of ancient beings with a unique purpose.
Perhaps a fanfiction where Grimlock is initially distrustful of Optimus Prime's more diplomatic approach. But as the story progresses, they learn from each other. For example, Grimlock could teach Optimus about the value of brute force in certain situations, while Optimus could show Grimlock the importance of negotiation. This would lead to a stronger bond between them and a more united Autobot force.