I got mixed feeling about this, its like letting go of someone who has been real close to you for 25 years. Ash and Pikachu were not only my first friends but my first teachers. I learned the word Pikachu before joining a school and started drawing the Pokémon cartoons. I am truly gonna miss these guys. Not sure, what the future holds for them. But, I would've loved to see Ash age and explore a little more, destroy team rocket once and for all. Have a final battle with Giovanni. Catch Ho-oh and literally become a Pokémon Master. Right now, he is a Champion, strongest in the world but being a Pokémon Master is not only about being the strongest trainer, its all about understanding Pokémon and respecting their feelings. In a sense, Ash is already a Pokémon Master. There have been times when Ash kept his feelings aside and let his Pokémon choose for themselves. This is what a true Pokémon Master is.
In my opinion, Ash has always been a Pokémon Master, and now he is The World Champion. There couldn't have been a better way to end Ash and Pikachu's journey. This is the end of an era. You will always be missed. No one can replace you!