Well, it could be about Mewtwo, a powerful psychic Pokémon, and Arceus, the creator Pokémon, having a battle of wits. Maybe Mewtwo challenges Arceus' authority in the Pokémon world, leading to an epic showdown where their powers clash and the very fabric of the Pokémon universe is at stake.
Perhaps the main antagonist is an entity that represents chaos and destruction itself. It could be a force that is awakened by the meeting of Mewtwo and Arceus, seeing their combined power as a threat to its existence, and thus tries to stop them from achieving whatever goal they have in the fanfiction.
One interesting plot could be Ash somehow getting a power - up that allows him to communicate with Arceus on an equal level. Maybe he discovers a new type of Pokémon energy that gives him this ability. Another plot could be that Arceus decides to test Ash to see if he's worthy of some great power or responsibility in the Pokémon world.
In a fanfic, Ash might have a chance against Arceus. Ash has proven time and again that he can adapt to different situations. He could potentially find a Pokémon's weakness or use some unorthodox strategy. However, Arceus is the creator of the Pokémon world. It has the ability to manipulate reality on a fundamental level. So, on paper, Arceus should win. But fanfics often play with the rules, so Ash could win through some plot - driven miracle.
One possible plotline could be that the Lake Trio discovers a threat to the world that Arceus is either unaware of or is causing (unintentionally perhaps). So they have to battle Arceus to stop this threat. Another could be that a powerful villain manipulates the Lake Trio into believing that Arceus is a danger and they must fight it.
It's hard to say for sure. Arceus is super powerful as the creator Pokemon. But the Creation Trio also has some really strong abilities. So it could go either way depending on how the fanfic is written.
Mewtwo is significant as it was created through genetic manipulation. It was made to be a powerful Pokémon. In the 'Mewtwo Origin Story', it discovers its origin and questions its purpose, which is a major part of its significance.
In the 'pokemon mewtwo origin story', it was a group of scientists who created Mewtwo. They were trying to make a very powerful Pokemon by cloning Mew.
In a'mewtwo meets mewtwo fanfiction', it could start with the two Mewtwos being summoned by an unknown force. At first, they are confused and defensive. As they interact, they find out that their memories of creation and the pain of being experimented on are very similar. They might then decide to search for the source of this unknown force together. Along the way, they face various challenges, like other powerful Pokémon trying to stop them or humans who fear their combined power. But through their shared experiences and newfound bond, they are able to overcome these obstacles.