Another possibility is Bulbasaur. It has always been a loyal and steady companion to Ash. During the trip, it could see how far they've come together and confess its gratitude for all the experiences and growth they've shared.
Well, in such a fanfiction, perhaps Pikachu might be on this pokemon trip and confess its deep friendship and loyalty to Ash. It could say how much Ash means to it and how their adventures together have been the most important part of its life.
Pikachu is a must - include. It's Ash's most iconic Pokemon and always adds a lot of charm and power to the story. Brock could also be great as he has a lot of knowledge about Pokemon and can offer valuable advice.
Gary Oak is a likely character. He has always been a rival to Ash and in fanfiction, he might betray Ash out of jealousy or a desire to be the ultimate Pokémon trainer. Another could be Misty. Maybe she gets influenced by some external force or her own insecurities and turns against Ash. Brock could also be a candidate in some stories. If he falls in love with a girl who has a vendetta against Ash, he might be forced to choose between his loyalty to Ash and his new love interest.
He'll surely meet the Alola region's professors. Also, the local trial captains are very likely to cross paths with Ash. And of course, the regular Pokémon trainers in Alola.
Gary could be a likely character. He has always been Ash's rival and there could be a plot where his jealousy or some other motive leads him to betray Ash. Another could be one of his so - called friends from the Pokemon League who might be influenced by power or some dark secret. Or, in some really dark fanfictions, it could be a Pokemon that Ash trusted deeply but for some reason turns on him.
Ash is a well - known Pokémon trainer, always eager to catch new Pokémon and battle strong opponents. He has a kind heart and is very passionate about Pokémon. Dawn is another trainer, who is often shown to be determined and has her own unique style of training. In the 'dawn confesses to ash fanfiction', their relationship takes a turn towards the romantic as Dawn decides to be honest about her feelings for Ash.
Maybe Dawn could be Ash's date. They have a great friendship and have journeyed together. Their knowledge of Pokemon contests and battles could be a fun part of their date during the trip. Also, Anabel from the Battle Frontier is a possibility. She's a strong trainer, and Ash has had interactions with her. A date with her during a trip to the Battle Frontier or another region could bring some new and exciting dynamics to the story.
Team Rocket is very likely. They've always been after Ash and his Pokémon. Using mind control on Ash would be a new way for them to get what they want. They could make Ash give them Pikachu or other strong Pokémon. Another one could be Giovanni. As the leader of Team Rocket, he has the resources and the motive to use mind control on Ash to further his own evil plans in the Pokémon world.
Ash is definitely one of the main characters since he's the one being saved. His Pokémon, especially Pikachu, are likely to be main characters too as they would probably be involved in the saving process.
Brock could be an interesting secondary character. His knowledge of Pokemon and his role as a sort of mentor figure could add depth to the story. He might give Ash and Trip advice on Pokemon care or training.