Arcee might also be a candidate. She is a strong and independent Autobot. A fanfiction could depict a love story where Optimus Prime and Arcee start off as comrades in arms and gradually develop deeper feelings. Their relationship could be complicated by the constant threat of war and the need to make sacrifices for the greater good of the Autobot cause.
In some fanfictions, a human female character could be Optimus Prime's love interest. For example, a scientist who works with the Autobots. She could understand Optimus on a different level, seeing his noble qualities beyond his robotic form. Their love story could involve her helping him understand human emotions better, while he protects her from the dangers of the Transformers' battles.
A strong - willed female character could be his ideal partner. Maybe someone like Yuffie. She is brave and has her own set of skills. Her lively personality could contrast with Vincent's brooding nature, and she could bring out a different side of him in the love story.
A possible love interest could be another Autobot. Maybe a female Autobot with whom he has fought side by side for a long time and gradually developed feelings for. Another option could be a human female who has shown great courage and understanding, and through their interactions, Optimus Prime finds himself attracted to her.
Another option might be Robin. She has a calm and intelligent demeanor. Robin and Luffy share a mutual respect. Her knowledge and Luffy's leadership could make for an interesting dynamic in a marriage within the fanfiction world. Also, they both have a common goal of finding the One Piece in a way.
I think Yuki could be an interesting partner for Akatsuki Kain. Their different personalities, with Yuki's kindness and Akatsuki's more complex nature, could create a lot of interesting dynamics. They could help each other grow and face the challenges in the vampire world together.
A strong female Autobot would be a great love interest. They already share the same values and goals, and the bond of being on the same side in the fight against the Decepticons could easily turn into a deeper emotional connection.
Hermione could be his ideal partner. She has known Harry for years and his newfound (in the fanfiction) tall and handsome features might just make her see him in a different light. Their shared experiences and her intelligence would complement his good looks and height well.
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 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.
Han Solo could be an ideal crossover partner for Thorn. Han is a skilled pilot and a bit of a rogue. He and Thorn could get into all sorts of adventures together, smuggling goods or escaping from the Empire. Another good partner could be Ahsoka Tano. She has great combat skills and knowledge of the Force. Thorn could learn a lot from her, and they could fight side by side against common enemies.
One possibility could be that the skateboard girl discovers Optimus Prime in a hidden alley. Maybe she's skating at night and sees a strange glow. She approaches it and finds Optimus Prime. She then becomes his helper, using her skateboarding skills to quickly get around the city and gather information for him. She could also teach him about the local youth culture, which he finds fascinating.