One great 'Ash go to Altomare' fanfiction could be 'Ash's Altomare Adventure'. It might have Ash exploring the unique features of Altomare, meeting new Pokémon and people. Another could be 'Ash in Altomare: A New Beginning' where he gets involved in some local events in Altomare and forms new friendships. And there's also the fanfiction 'Ash's Journey to Altomare' which could focus on his travel to the place and the challenges he faces along the way.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). Just type 'Ash go to Altomare' in the search bar on those sites. There might also be Pokémon - specific fanfiction communities where you can ask for recommendations on 'Ash go to Altomare' fanfictions.
He could go to the mountains. There are so many hidden caves and areas in the mountains that he could explore. It would be a great place for him to hide and be alone with his thoughts. Plus, there might be some powerful Pokémon that live there that he could interact with.
The time when Ash and May go to Kalos together in fanfiction is completely up to the creator's choice. It could be at any point in the story to serve the intended storyline and character development. Sometimes, it might be a sudden decision that leads to new challenges and discoveries, and other times, it could be a well-planned event that ties into the overall theme of the fanfiction.
Perhaps in this fanfiction, Ash goes to the town in search of a rare Pokemon item. But when he arrives, he discovers that Latias and Latios are in trouble because of some evil forces. So, Ash decides to help them out. He uses his battle skills and knowledge of Pokemon to fight against these forces. Along the way, he also learns about the unique culture and traditions of the town.
In Alola, Ash and Serena will have a great time. Ash will be eager to learn about the Z - moves and how they can be used in battles. Serena, on the other hand, will enjoy the local cuisine and perhaps use it as inspiration for her Pokemon contests. They may also meet some friendly Alola trainers who can show them around the islands and introduce them to the local Pokemon traditions.
They might have all kinds of adventures. They could explore the unique Pokémon in Alola. Ash would be excited to discover new Pokémon species and train them. Serena might be interested in the local culture and how it relates to Pokémon. Maybe they'll participate in some Alolan Pokémon contests together, with Serena showing off her skills and Ash cheering her on.
One possibility is that they get involved in a mystery related to a rare Pokémon in Kanto. They would have to work together to solve it. Ash's instincts and Serena's intelligence would be put to the test. Along the way, they could also visit some of the famous landmarks in Kanto like the Pokémon Tower. This journey would not only be about Pokémon but also about them getting to know each other better in a new environment.
The timing of Ash going across Kanto again after training in fanfiction is really up in the air. It's entirely at the whim of the person writing the story and could happen at any point they choose to make it happen.
In this fanfiction scenario, Ash and Charmander could have an adventure in a volcanic area. Since Charmander is a fire - type, it would feel right at home there. They might discover a new type of Pokemon that is only found in volcanic regions. Also, Ash could participate in a local fire - type Pokemon tournament in this area. With Charmander's growth and training, they could potentially win the tournament and gain some rare items or Pokemon eggs as rewards.