Tarantulas is an interesting character to include. He has his own devious plans that often intersect with Megatron's, sometimes as an ally and sometimes as a rival. Tigatron, with his connection to nature and his more spiritual side, can bring a different perspective to the story. And Waspinator, for his comical yet tenacious nature. Despite being often defeated, he always comes back, and his interactions with Megatron can be both humorous and revealing of Megatron's character.
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.
One great Bumblebee and Megatron fanfic could be about their unexpected alliance. Maybe in a universe where they have to team up against a greater threat. They start off distrusting each other but as time goes on, they learn to respect one another's skills.
To create believable character development for Megatron, consider his past. Maybe he had some experiences with humans in the past that were either positive or negative but were buried deep. In the fanfiction, these memories could resurface due to an encounter with a particular human. As he interacts more with this human, his views on the entire human race could start to shift. For instance, if he once had a human friend as a young Transformer but lost them in a war, meeting someone similar could reignite those old feelings and make him more empathetic towards humans.
If Naruto was a Megatron fan, he might become more assertive. Megatron is known for his strong leadership and determination. Naruto could adopt some of these traits, becoming more confident in making decisions and leading his team. He might also be more willing to take risks, like Megatron does in battle.
The Chinese zodiac was a tiger. Tigers were known as the king of beasts. They had wisdom and courage, and were the top predators in the jungle. Tiger Megatron's characteristics were leadership, talent, and decisiveness. As the leader of one of the twelve zodiac animals, the tiger could display its power in all directions. The image of the tiger was extended from the image of its leader.
The fanfiction might also explore the relationship between Bumblebee and Megatron in a more emotional way. Maybe Bumblebee discovers a softer side of Megatron that was hidden under all that aggression. It could be that Megatron has some regrets about his past actions, and Bumblebee becomes the one he confides in. Or perhaps Megatron sees in Bumblebee a potential future for Cybertron that he once hoped for but thought was lost. This could lead to a very unique and unexpected bond between the two characters.
Yes, in some fanfictions, Megatron can be depicted as younger than Optimus. Fanfic writers often take creative liberties with the characters' backstories and timelines.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often a plethora of stories involving these characters. Just use the search function and type in 'Megatron x Rodimus' and you'll likely get a good number of results.