One great fanfiction could be about Optimus and Arcee going on a dangerous mission together. They face numerous challenges, like battling a powerful new Decepticon. Their teamwork and trust in each other are put to the test. Arcee's speed and agility combined with Optimus' strength make them a formidable duo. Along the way, they might have some deep conversations about the future of their race and their place in the war.
Love and loyalty is also a big theme. Since it's a relationship - based fanfic, it often explores how they feel about each other. Maybe Arcee has to prove her loyalty to Optimus in a difficult situation, or Optimus shows his love for Arcee in a selfless way.
Well, to write Optimus Prime, think about his wisdom. He has a vast knowledge of their history and the universe. Incorporate this into the story when he's giving advice or making plans. For Arcee, explore her emotional side. Maybe she has a soft spot for a certain young Autobot. When they are in a dangerous situation, Optimus might be calm and calculating, while Arcee could be more impulsive, which can create interesting dynamics in the fanfic.
One possible fanfiction could be about Arcee and Optimus teaming up for a dangerous mission. They might face a powerful new enemy that neither of them has encountered before. Their combined skills and trust in each other would be crucial in overcoming the challenges. For example, Arcee's agility and Optimus' strength could be used in a complementary way to defeat the foe.
Perhaps a fanfiction where Arcee has to save Optimus Prime from a mind - controlling device. She has to fight against his altered self and find a way to break the control. This would show her determination and loyalty to Optimus.
You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. There are often a lot of Transformers fanfics there, and you should be able to find some great Optimus Prime x Arcee ones by using the search function.
Romance is also a theme, though it can be a bit tricky in the context of Transformers. Writers often explore the emotional connection between Arcee and Optimus Prime, how they might have a deeper understanding of each other that goes beyond just comradeship.
A great plot could involve Arcee getting injured during a fight and Optimus Prime having to protect her while she recovers. During this time, they could have deep conversations about their hopes for the future of their race. Or perhaps there's a fanfic where they discover an ancient artifact that has a connection to both of their pasts, and they have to work together to unlock its secrets.
The theme of sacrifice often appears in fanfictions about them. They are willing to put themselves in harm's way for the greater good. This could be shown in a scenario where they have to sacrifice a strategic position to save a group of innocent Autobots. Also, the theme of redemption can be explored, especially if a character from their past resurfaces and they have to decide whether to forgive or not.
Perhaps a fanfiction where Arcee and Optimus are transported to an alternate dimension. In this new world, they have to figure out how to get back home while also dealing with the strange new rules and inhabitants. They might find that their relationship is put to the test as they face unfamiliar and often dangerous situations. They could also learn new things about each other that they didn't know before.