For a 'rwby megatron fanfiction', Ozpin could be a main character. He has a lot of knowledge and could potentially figure out a way to deal with Megatron. The RWBY girls would also be main characters as they are the heroes of their world. Megatron would be the main antagonist, driving the conflict in the story with his desire for power and domination.
Often, other Decepticons like Starscream might be involved. Starscream, always vying for power, might see Megatron's condition as an opportunity to finally take over. So he could play a major role in the fanfiction, either causing trouble or having a change of heart. And then there could be some human characters who get caught up in the events, perhaps scientists or soldiers who are trying to understand what's going on with Megatron.
Obviously, the members of Delta Squad would be main characters. They'd bring their own personalities and abilities to the story.
New students from different kingdoms could also be main characters. For instance, a student from Vacuo who has a unique sand - based fighting style. Or a student from Atlas who is a tech genius and uses advanced weapons. They could team up with the descendants of Team RWBY to form a new, diverse team.
Well, obviously Megatron is a main character. In his human form, he's the center of the story. Often, there will be some human characters as well. Maybe a scientist who was involved in the process that turned him human, or a normal person who becomes his friend or ally.
Ruby Rose could definitely be one of the main characters. Her leadership skills and curiosity would make her interested in these aliens from Earth. She might be the one to initiate contact or try to understand their motives.
Obviously, Ichigo would be one of the main characters. He brings his unique set of powers and personality to the crossover. Ruby Rose could also be a central figure. Her leadership and scythe - wielding skills could interact with Ichigo in various ways, perhaps in combat or in making decisions for the group.
In this fanfiction, Sakura from Naruto could be a main character. She has medical skills and combat abilities that could be useful in dealing with the aftermath of the Kyuubi's rampage. Alongside her, Weiss from RWBY could be a main character too. Her ice - based powers might be used to try and cool down the Kyuubi's fiery rage.
One idea could be a crossover where Megatron somehow ends up in the RWBY universe. He might see the potential for power in the Dust and try to take control of it. Another idea is that the RWBY characters have to team up with an Autobot to stop Megatron from causing chaos in their world.
Ruby could be a main character. Her speed and scythe - wielding skills would be really interesting to see in the Titanfall universe. She could quickly adapt to piloting a Titan and use her scythe in new and creative ways during battles.
I'm not sure specifically as there are many possible interpretations in fanfiction. It could still feature Ruby, Weiss, Blake, and Yang as main characters, but with a plot centered around dilation. For example, Ruby might be the one to first notice something strange related to dilation and lead the team to investigate.