In a fanfiction, it could be that there is a threat to the very fabric of their universes. Optimus Prime and Optimus Primal are brought together by a mysterious force. They start exploring each other's worlds, learning about the unique challenges each face. Along the way, they meet other characters from both universes who either help or hinder their quest to save both their worlds. This would allow for a lot of character development and exploration of the two different Transformers universes.
The plot might involve a misunderstanding at first. Optimus Prime, coming from the Autobot world, and Optimus Primal, from the Beast Wars world, could have different ways of doing things. But as they get to know each other, they realize they share the same values of protecting their respective worlds and decide to work together. For example, Primal could teach Prime about the advantages of using a more beast - like form in certain terrains, and Prime could share his knowledge of advanced technology with Primal.
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.
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.
In such a fanfiction, Naruto could be fascinated by Optimus Prime's technology. He might try to find ways to incorporate some of the Autobot's tech into his ninja tools. Maybe he'd develop a new type of shuriken that has some of the functions of Optimus' weapons. Also, Naruto could learn about the concept of a team from Optimus, where every member has a role to play, and apply it to his own ninja team dynamics.
One possible plot could be that during a dangerous mission, Optimus Prime and Arcee get separated from the rest of the team. As they struggle to survive and find their way back, they rely on each other more than ever. This shared experience deepens their bond, and eventually, they share a kiss as a sign of their new - found closeness.
The plot could be about a power struggle. Megatron wants total control over Cybertron, but Optimus Prime stands in his way. However, instead of just fighting, there are political intrigues involved. There could be other characters influencing their decisions. Maybe some Autobots who think Optimus is being too soft on Megatron, and some Decepticons who think Megatron is being too cautious.
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.
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.
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.
Baby Optimus Prime's character can be developed by exploring his relationships. If he has a mentor figure among the Autobots, his interactions with that character can shape him. He might look up to this mentor and try to imitate their good qualities. His relationship with other baby Transformers or younglings can also be important. He could learn about sharing, cooperation, and friendship through these relationships. Additionally, his discovery of his own strength, both physical and mental, can be a key part of his development. For instance, when he first realizes he can lift a small object that others thought was too heavy for him, it gives him confidence.
The other characters in the Highschool DxD world would have a wide range of reactions. The angels might see Optimus as an unknown factor and be a bit suspicious. The fallen angels might try to manipulate the situation to their advantage. And the normal human students would be terrified at first. But as the story unfolds and they see Optimus Prime's noble actions, they could start to look up to him as a hero, much like how the Autobots are seen as heroes in their own universe.