One idea could be Ash getting a special upgrade to his Mega Buster that allows him to shoot different types of elemental blasts. For example, fire blasts to defeat ice - type enemies more easily.
A really cool fanfiction could involve Ash traveling to a parallel universe where the Mega Buster is a common weapon among Pokémon trainers. There, he has to learn new techniques and strategies with the Mega Buster to compete in battles and find his way back home.
A new villain, a mad scientist type, who is obsessed with the power of the Mega Buster. He wants to reverse - engineer it to create an army of robotic Pokémon with similar weapons.
One idea could be that Buster Moon discovers Ash has a secret talent that she's been hiding due to her tough exterior. For example, she can write amazing songs but is too shy to share them. Buster then encourages her to open up and they collaborate on a song that becomes a huge hit in their world.
One interesting plot could be that Charizard's mega evolution causes it to lose some of its memories. Ash then has to go on a journey to help Charizard regain those memories while also dealing with the challenges that come with having a mega - evolved Pokémon. For example, other trainers might be jealous and try to challenge Ash more often. Also, Charizard's new power might attract the attention of powerful Pokémon that want to test their strength against it.
Well, 'naruto mega buster fanfiction' could be about a character in Naruto who discovers or creates a mega buster. This could lead to all sorts of new storylines. For example, they might use it to fight a powerful new enemy, or it could cause some sort of conflict among the ninja clans because of its power. Maybe it's a story that focuses on how this mega buster changes the balance of power in the Naruto world and how the different characters react to it.
Perhaps there's a story where Ash travels to a new region where Mega Garchomp are rare and feared. The local people initially oppose Ash having a Mega Garchomp with him. But through a series of events, like Mega Garchomp helping a local village with their crops by using its power to control the weather, the people start to accept them. And in the end, Ash and Mega Garchomp become local heroes.
I don't know for sure as it's a fanfiction. It could be Naruto himself using the mega buster. Or it could be a new character created specifically for this story.
One idea could be that Buster and Montana Max are forced to work together on a school project. Despite their differences, they start to understand each other better. Buster realizes that Max isn't all bad, and Max sees Buster's cleverness as an asset.
Another interesting plot could be a crossover with a post - apocalyptic world. The Monster Buster Club members find themselves in a world where the monsters have already taken over. They have to team up with the survivors of that world, share their knowledge of fighting monsters, and find a way to reverse the situation and restore peace.
One idea could be Naruto finding the Buster Sword in a hidden ancient ninja cache. He then has to learn how to use it in his own unique style, combining his chakra powers with the brute force of the sword.
Well, a plot could revolve around a young inventor in a parallel universe who discovers a way to communicate with the original Mega Man. Together, they try to find a solution to a virus that is spreading across multiple Mega Man universes, corrupting all the robotic beings. This would involve lots of action, as they have to fight their way through hordes of corrupted robots while also searching for a cure.