Metro Man might be more laid - back about it. He'd probably see it as a fun take on their world and might even chuckle at some of the more outlandish interpretations of their battles and relationships. He could also use it as a chance to better understand how the public perceives them both.
Perhaps he gets a power related to mind control. This would allow him to influence the actions of his enemies or even make them fight among themselves. He could also gain a power that lets him absorb the powers of his defeated foes. So, every time he defeats an enemy, he becomes stronger by adding their powers to his own arsenal.
I'm not entirely sure without more context, but he is likely the central figure in this story. He might be someone who has had extraordinary experiences in facing life's challenges directly.
Obviously, Dipper is the main character as the title implies. His sister Mabel is often included too. She might react in various ways to Dipper getting powers, like being excited or a bit jealous. Wendy is also a common character. She could be a source of support or a love interest for Dipper in some fanfictions.
In this fanfiction, Robin might get powers as a result of a genetic mutation triggered by a rare chemical. These powers could include enhanced speed and the ability to see in the dark. He initially hides his powers from the team because he's afraid of how they'll react. But when a dangerous situation arises where his powers could be the key to victory, he reveals them and they all work together to defeat the enemy and accept his new status.
Well, it could be that Naruto gets some kind of elemental powers from the comet. Maybe the comet is associated with a new, rare element that Naruto can then manipulate. This new power could give him an edge in battles, as his opponents would be unfamiliar with it. He could use it in creative ways, like combining it with his existing jutsu to create even more powerful attacks.
Master Chief could potentially gain the power of super speed in fanfiction. Imagine him being able to zip across battlefields in the blink of an eye, dodging enemy fire and quickly reaching objectives. This would make him an even more formidable force on the battlefield.
Well, the plot could involve Marco suddenly getting super strength. He might start by accidentally breaking things around him. Then, he could learn to control his power and use it to protect his friends from some sort of villains in his town. Maybe there's an evil wizard causing trouble and Marco, with his newfound strength, is the only one who can stop him.