Well, Megatron's personality in G1 fanfiction can be very domineering. He is the leader of the Decepticons and acts like it. He is not afraid to punish those who oppose him, especially Starscream. Starscream is usually painted as a conniving character. He is always looking for ways to gain power. He may seem sneaky, but at times, he can also show a certain level of intelligence. In some fanfictions, he is able to outsmart Megatron briefly, but his lack of real leadership skills always catches up with him. He is more interested in personal gain than the overall success of the Decepticons.
Well, there's a fanfiction where Starscream discovers a hidden power and tries to use it against Megatron. It's called 'Starscream's Secret Weapon'. In this story, the author really plays up the tension between the two characters. Also, 'The Rivalry Continues' is a popular one. It's set during the G1 era and shows how their constant power struggle affects the Decepticon cause. They are constantly at each other's throats, but also realize they need each other in a way.
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.
Well, I think Bumblebee would have a shot at winning in a battle of wits. Bumblebee has been shown to be very adaptable. In a fanfic, he could use his knowledge of Megatron's weaknesses, like his overconfidence. Bumblebee might pretend to be more vulnerable than he really is, luring Megatron into a false sense of security. Then, when Megatron least expects it, Bumblebee could strike with a clever plan. Megatron, on the other hand, is used to being in control and might not expect such a devious tactic from Bumblebee.
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.
A really dramatic plot twist could be if Bumblebee and Megatron had to team up in the fanfic. Maybe there's a greater threat that neither of them can face alone. It would be so unexpected because they're enemies. Bumblebee would have to put aside his hatred for Megatron, and Megatron would have to trust Bumblebee at least a little bit.
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.