When he travels to a time before the war started on Cybertron, he might be more idealistic. He could be seen as someone who is still fighting for what he believes is right for Cybertron, rather than the power - hungry dictator he becomes in the main timeline. He may be more open to negotiation and less likely to immediately resort to violence.
If Megatron travels back to a time when he was just a gladiator, he might be more brash and less calculating. His focus would be on survival and rising to power through brute force.
In a future time period, if he somehow travels forward, Megatron could be either more jaded or more desperate. If the Decepticons have lost the war in that future, he could be desperate to change the past to avoid that outcome. Or if they won, he could be jaded from having achieved his goal but finding it unfulfilling, and thus might try to go back and change things in a different way.
If Megatron travels through time, he might become more cautious. Having seen the consequences of his past actions, he may be less impulsive. For example, instead of rushing into battles, he'd plan more carefully.
One plot idea could be Megatron traveling back in time to the very beginning of the Cybertronian war. He could try to change the initial strategies that led to his early defeats, using his knowledge of future events. Maybe he'd attempt to eliminate Optimus Prime before he becomes a major threat.
One possible plotline could be Megatron traveling back in time to prevent his own creation. This would create a paradox as his actions in the present are a result of his past self. Another could be him going to a time when the Autobots were at their weakest and trying to finish them off once and for all.
If Natsu travels to a time when Fairy Tail was just starting, he would be more eager to prove himself. His competitive side would be even stronger. In a more distant past, like when dragons were still around, he might be more in awe and a bit more reserved as he encounters these powerful and mysterious creatures. In a future where magic has evolved, he could be a bit out of place at first, but then adapt and become more open - minded to new forms of magic.
One possible fanfiction could be about Optimus Prime and Megatron having to team up against a greater threat. Maybe an ancient evil from Cybertron's past resurfaces that is so powerful that their usual enmity has to be put aside. They start off distrusting each other but as they face battles together, they gradually come to respect the other's fighting skills.
A fanfiction could focus on a time when Megatron and Soundwave are separated. Maybe Megatron has been captured by the Autobots, and Soundwave is left to lead the Decepticons in his absence. Soundwave, who is not used to being in the spotlight, has to make difficult decisions. He has to figure out how to rescue Megatron without sacrificing too many of their troops. Meanwhile, Megatron is facing his own challenges in captivity, perhaps having to deal with Optimus Prime's attempts to make him see the error of his ways. This situation could create a lot of tension and drama in the fanfiction.
Fanfiction about them can explore different universes and scenarios. For example, what if they were raised together? How would their relationship be different? It gives fans a chance to play with the established lore and create new and exciting stories that the official Transformers storylines may not explore. It also allows for deeper exploration of their motives and personalities.
In the past, Luffy would likely be even more of a novelty. His fearless and somewhat naive attitude would stand out in an era where pirates were perhaps more cutthroat and calculating. He might bring a sense of hope and a different way of thinking to those around him. Law, in the past, could be seen as a bit of an enigma. His quiet and strategic nature might make him seem like a dangerous wildcard. In the future, Luffy could struggle to adapt to new social norms, but his unwavering loyalty and determination would still shine through. Law might be more intrigued by the future's medical advancements and would be eager to learn and incorporate new knowledge into his own skills.
His combat abilities might be overpowered in some earlier time periods where the fighting styles are less developed. For example, if he travels back to a time when lightsaber combat was more rudimentary, his advanced techniques could make him seem like a god of war.
Natsu, being his impulsive self, might initially have a hard time adjusting to more reserved time periods. For example, in a time when magic was more secretive and not used so openly, he might constantly be getting into trouble for showing off his powers. Ultear, on the other hand, who is more calculating, might find it easier to blend in. But in a more chaotic time period, like during a war, Ultear's calm demeanor could be shaken as she has to face more extreme situations. In a peaceful future time, Natsu might become more laid - back as there's no need for constant fighting, while Ultear might feel a bit out of place without her usual problems to solve.