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.
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.
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.
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.
A strong female Autobot would be a great love interest. They already share the same values and goals, and the bond of being on the same side in the fight against the Decepticons could easily turn into a deeper emotional connection.
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.
One interesting Optimus Prime love fanfiction could be about his relationship with a human female who helps him understand human emotions better. In such a story, they might go through various challenges together while fighting the Decepticons and in the process, their bond deepens.
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.
One possible fanfiction could be about Sentinel Prime and Optimus Prime having to work together to save Cybertron from a new, powerful enemy. They might start off with some differences in their approaches but gradually learn to combine their strengths.
A plotline could be Sentinel Prime getting corrupted by dark energy and Optimus Prime having to fight him to bring him back to the light. It would be a battle of wills and also a test of their long - standing friendship.
A plotline could be Dark Optimus Prime's redemption. Maybe he realizes the error of his ways after causing a great deal of destruction. This could involve him facing his inner demons and seeking forgiveness from his former comrades.