First, you need to understand Optimus Prime's character. He is a noble leader, so make sure his actions and decisions reflect that. For example, he would always put the safety of his team and the greater good first. Then, come up with an interesting plot. Maybe it could be about a new Decepticon threat or a journey to a mysterious planet. Also, include some great action scenes like epic battles between the Autobots and Decepticons.
The reader could be an ordinary human who just happens to be in the right place at the right time. Maybe they stumble upon Optimus Prime during one of his difficult moments and use their wits and human creativity to assist him.
In a fanfiction, Ratchet and Optimus could be sent on a secret mission. They are alone, away from the rest of the Autobots. This gives them a chance to have more personal conversations. They might discuss their hopes for the future of Cybertron, their fears about the Decepticons, and their memories of the old days. The story could show how they support each other not just in battle, but also on an emotional level.
Well, it could be a fanfic that involves the character Optimus Prime in a zone called Shadowzone. Maybe it's about some adventures or challenges he faces there. Without reading it, it's hard to say exactly.
Another idea is for a time - travel fanfiction. Ratchet and Optimus are sent back in time to an important moment in Cybertron's history. They have to be careful not to disrupt the timeline too much while also trying to change certain events to prevent the rise of the Decepticons. This would require them to work together closely, using Ratchet's knowledge of history and Optimus's leadership skills to navigate this dangerous new situation.
The plot could involve Optimus Prime facing some sort of internal struggle. Maybe he's having trouble dealing with the new threats in his world and a reader, who could be a human or another Cybertronian, gets involved in his story. They might start to understand his problems and try to help him out.
In the official Transformers lore, there is no canonical love story between Arcee and Optimus Prime. Their relationship is mainly based on comradeship within the Autobot ranks. They are both focused on the greater mission of protecting Cybertron and the universe from the Decepticons.
In some fanfictions, Soundwave might be malfunctioning and Optimus tries to help him. This creates a unique situation as they have to put aside their differences. Their interaction could show a different side of their characters, like Soundwave's vulnerability and Optimus' compassion outside of the normal Autobot - Decepticon conflict.
You could try looking on fanfic websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fanfics, so there's a chance you'll find it there.
Well, it could be about a reader who somehow gets connected to Vector Sigma and meets Optimus Prime. Maybe the reader has a special ability that Vector Sigma detects and Optimus Prime needs to protect the reader from the Decepticons who also want to use this ability for their evil plans.