Perhaps there's a fanfiction where the humans are using bath bombs in a large bath tub they've set up near the Autobot base. Optimus and Ratchet come across this strange activity. The humans explain that it's a way to unwind. Optimus, being the diplomatic leader, decides to learn more about this human custom. He asks the humans various questions about bath bombs, like what they are made of and why they fizz. Ratchet is more concerned about the practicality but eventually gets drawn into the fun as the humans start sharing stories about how much they love bath bombs.
They could be incorporated by having the humans use bath bombs as a form of celebration. For example, when the Autobots achieve a major victory, the humans decide to have a party and use bath bombs as part of the celebration. Optimus and Ratchet are invited to join. Optimus might see it as a way to bond with the humans further, while Ratchet would be more interested in the scientific aspect of how the bath bombs work.
One idea for this fanfiction could be that a group of humans who are fans of the Transformers create special bath bombs with the images and symbols of Optimus and Ratchet on them. When they use these bath bombs, they accidentally summon the real Optimus and Ratchet. At first, there's chaos as the two Autobots are confused about where they are. But then, they start to bond with the humans over the shared experience of the bath. The humans show them how the bath bombs fizz and release colors and scents, and Optimus and Ratchet in turn share some of their Cybertronian knowledge about similar substances from their home world.
Optimus might be very polite and respectful in his interaction with humans in this fanfiction. He could see the use of bath bombs as a unique human way of relaxation. Ratchet, on the other hand, might be a bit more skeptical at first. He could be like 'What is the point of these fizzy balls in the water?' But as he sees the humans enjoying it, he might start to understand the concept of self - care. They both could learn a lot about human emotions and the importance of taking time for oneself from this experience with the bath bombs.
Another theme is the idea of a deeper connection. It could be a romantic or a very deep platonic connection. Many fanfics explore how they understand each other on a level that others in the Autobot team may not. They share glances, have private conversations that show the special nature of their relationship.
There's also a fanfic called 'Healing Hearts'. In this story, Ratchet plays a crucial role in mending Optimus after a particularly difficult battle. The author does a great job of exploring their friendship that could potentially be more. It has some really touching moments that make it a great read for fans of the pairing.
Another aspect is the acceptance of differences. The human reader has to accept that Optimus Prime is not human, he has different values and a different physical form. Optimus Prime has to accept the fragility and short lifespan of the human. They need to work through these differences and find common ground. For instance, they both value justice and freedom. By focusing on these shared values and gradually understanding their differences, their relationship can develop in a believable way.
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.
A fanfiction might involve Optimus getting injured in a battle. Ratchet then has to use all his skills to save him. During the recovery process, they have deep conversations about the war, their hopes for the future, and the sacrifices they've made. It could explore their different perspectives as leader and medic.
There could be a fanfiction where Optimus and Ratchet are stranded on an alien planet. They have to work together to survive, find resources, and figure out a way to contact the other Autobots. This would really highlight their survival skills and their ability to cooperate under extreme pressure.