One reason could be his sense of justice. Ash has always been about doing what's right in the Pokémon world, so saving all the legendaries would be a natural extension of that. Another reason might be his love for Pokémon in general. He wants to protect all Pokémon, and legendaries are no exception.
In a lot of fanfiction, Ash is indeed portrayed as the one who saves all legendary Pokémon. This concept can be really thrilling for fans. Imagine Ash, with his Pikachu by his side, facing all kinds of challenges and foes to rescue these powerful and often mysterious Pokémon. It gives a sense of epic adventure. Also, it can be a way for writers to explore different aspects of the Pokémon lore and the relationships between different legendary Pokémon and Ash. It makes for some very engaging reading, but again, there are also fanfictions that deviate from this idea.
In many fanfictions, Ash is indeed portrayed as having all the legendary Pokémon. This can add an exciting twist to the Pokémon universe. It allows for the exploration of different relationships between Ash and these extremely powerful creatures. For instance, he could use their combined powers to face off against a new, even more menacing threat. It also gives a chance to see how the Pokémon world would react to someone having such an extraordinary collection. Some fanfictions might even have Ash use the knowledge and abilities of the legendaries to teach other trainers or to make the Pokémon world a better place in terms of understanding different species better.
In most fanfictions, it's a creative and exciting concept. However, in the official Pokémon lore, it's highly unlikely. But in this particular fanfiction, it could be possible. It depends on how the author has written the story and the rules they've set within that fictional world.
One great fanfiction about Ash as a legendary Pokémon trainer could involve him traveling to a new, mysterious region filled with rare and powerful Pokémon. He might start with his Pikachu, of course, and gradually build a team of Pokémon that are not only strong but also have unique abilities.
Mewtwo is also a common Legendary Pokémon in Ash - related fanfiction. Mewtwo's complex personality and its powerful psychic abilities make for interesting interactions with Ash. There can be stories about Mewtwo respecting Ash's bond with his Pokémon and joining him on a journey, or even testing Ash's worthiness.
Yes, in some fanfics Ash might be depicted as getting all the legendary Pokémon. Writers often like to give him these powerful Pokémon to create interesting and unique storylines.
One implication is that he would be extremely powerful. He could easily dominate Pokémon battles and tournaments.
If a legendary Pokémon joined Ash in fanfiction, it would open up a whole new world of possibilities. First of all, Ash would need to learn how to bond with this extremely powerful and often independent - minded Pokémon. It might have different needs and a much grander sense of purpose compared to his regular Pokémon. Also, their battles would be epic. With a legendary Pokémon by his side, Ash could take on the toughest of Pokémon Leagues and might even be able to face threats that are endangering the Pokémon world directly. He could travel to new and mysterious places that are related to the legend of the Pokémon, uncovering secrets and having adventures that were previously unimaginable.
One possible plot twist could be that the legendary Pokémon start to lose their powers once caught by Ash. Another could be that there is a hidden cost or consequence to catching all of them that Ash doesn't initially realize.
In fanfiction, it could be written that a prophecy foretells of a trainer (Ash) who will be joined by a legendary Pokémon to save the Pokémon world. Ash unknowingly fulfills the conditions of the prophecy, and the legendary Pokémon appears before him. Then, through a series of trials and mutual understanding, they become partners on Ash's journey.