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.
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.
In a different scenario, Voldemort could be in search of a powerful new weapon and discovers a way to control Optimus Prime. Harry and the Order of the Phoenix have to find a way to break the control and convince Optimus to join their side. They could use their combined powers of magic and technology to defeat Voldemort once and 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.
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.
In a fanfiction, a Harry Potter and a Khal interaction could be full of misunderstandings at first. Harry's use of a wand and strange words for magic would seem very odd to the Khal. But as they get to know each other, they might find common ground in their leadership skills. Harry has led his friends in many dangerous situations in the wizarding world, and the Khal is a natural leader among his people. They could exchange ideas on how to lead and protect their respective groups.
One interesting concept could be a magical transformation of Optimus Prime. Maybe he gets enchanted by a Hogwarts spell and gains new abilities that combine his robotic powers with magic. For example, his laser blasters could be enhanced with a spell to have a freezing effect like the Glacius spell in the Harry Potter universe.
One way to blend the two universes is through the use of magical artifacts and Cybertronian technology. Maybe the Philosopher's Stone could be used to power up Optimus Prime in some way, or a Cybertronian device could be used to enhance a wizard's magic. This creates a connection between the two universes where they rely on each other's unique elements.
Optimus Prime's strength and ability to transform could be combined with RWBY's unique semblances. For example, Ruby's speed semblance could be enhanced when she rides on Optimus in his vehicle form. Or Yang's strength semblance could be amplified when she teams up with Optimus in a fight, using his power as a boost.
Perhaps it could be due to a cybernetic virus. The virus was designed to corrupt Autobot systems, but instead, it mutates Optimus Prime's transformation sequence. So instead of his normal truck form, he turns into a dragon. His dragon form would have a long, spiky tail that can be used as a weapon or for balance during flight.
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.