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 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.
They could learn a great deal from each other. Harry could teach Optimus about the different kinds of magic, like the Patronus charm which is used to fight off Dementors. Optimus, on the other hand, could show Harry the value of technology and teamwork within the Autobot ranks. Their combined forces could be a great asset in a fight. If they were facing a common enemy, say a new dark wizard who has also allied with the Decepticons, they could use both magic and advanced weaponry to take them down.
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.
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.
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.
The ponies' reactions would vary greatly. Rarity might be fascinated by Optimus Prime's design and might even try to make some sort of fashionable accessory inspired by him. Applejack would be more practical and would wonder what he was doing in Equestria and if he had any good intentions. Optimus would see the ponies as a new and interesting species. He would be patient with their initial wariness of him and would try to show them that he means no harm. He might also be a bit taken aback by their colorful and somewhat chaotic world compared to the more structured world of the Autobots.
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.
When it comes to making believable character interactions in an Optimus Prime crossover, you need to consider the cultural backgrounds of the characters. If it's a crossover with, say, a character from a feudal Japanese anime like Naruto. Optimus Prime's code of honor and his high - tech world would contrast with Naruto's ninja world of chakra and clans. Have them misunderstand each other at first, but then as they learn about each other's values, they start to work together. For instance, Optimus could help Naruto protect his village from a Decepticon - like threat, and Naruto could show Optimus the power of friendship and determination in his own unique way. This way, the interactions feel natural and believable.
A love story for Optimus Prime could involve a love at first sight with an intelligent and brave female Autobot. They would start to communicate more during battles and strategy meetings. Their love would grow as they rely on each other for support.
In time travel fanfics, Optimus Prime often becomes more introspective. For instance, when he meets his past self or witnesses historical events, he reflects on his past decisions. This makes him more cautious in some cases, as he realizes the far - reaching consequences of his actions.