One possible plot could be that they team up to fight a common enemy. Maybe a new, powerful Decepticon or Predacon that neither could defeat alone. Their combined strength and different abilities would make for an exciting battle.
At the beginning, they might be cautious around each other. Optimus Prime, being a more traditional Transformer, might find Optimus Primal's more organic and beast - like nature a bit strange. However, as they communicate, they would find common ground in their goals of protecting their people and planets. They could exchange stories about their battles and leadership experiences. For instance, Primal could tell Prime about how he led his Maximals in the Beast Wars, and Prime could share his tales of fighting the Decepticons. This exchange would help them build a bond and understand each other better.
One common theme is friendship. They have a strong bond in the movies, and in fanfiction, it's often explored more deeply. Another theme could be adventure. They might go on new missions together that weren't in the original movies.
First, you need to establish their relationship clearly. Think about what makes Optimus possessive of Sam. Is it because of Sam's unique role in the Autobot - human alliance? Then, create some situations that highlight this possessiveness. For example, when other humans or Autobots try to come between them.
One popular 'possessive Optimus Prime x Sam fanfiction' might involve Optimus being overly protective of Sam. For example, in some stories, Optimus could use his power and influence to keep Sam out of dangerous situations that the other Autobots might not be as concerned about. It's a unique take on their relationship that adds an interesting layer of drama.
The plot could be really interesting. Maybe Optimus Prime, being from a world of war and machinery, would be a bit out of place in the colorful and magical world of MLP. He might initially be seen as a threat by the ponies, but as they get to know him, they realize he's there to protect. For example, he could help the ponies fight off some new dark force that neither his world nor theirs had faced before, combining his strength and strategic thinking with the ponies' unique magic powers.
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.
Well, perhaps in this fanfiction, Sam somehow ends up in the Transformers Prime universe. He might form a special bond with one of the Autobots, like Bumblebee. Together, they could go on adventures to stop the Decepticons from getting their hands on a powerful artifact. Sam's human ingenuity combined with the Autobot's strength would make for an exciting story.
Once upon a time, Optimus Prime was resting after a long day of protecting the universe. He parked himself in a quiet corner of the Autobot base. His optics dimmed as he thought about all the battles he had won and the friends he had made. The other Autobots moved quietly around him, knowing he needed his rest. And as he drifted off to sleep, his dreams were filled with visions of a peaceful future for all.
Optimus Prime could fit in as a powerful ally. He would be like a super - powered being in the RWBY world. His presence could shake up the power balance. Maybe he arrives through a portal and immediately starts helping the RWBY teams fight the Grimm. He could be seen as a symbol of hope, much like the Huntsmen and Huntresses are in RWBY.