A great storyline could be Serena using her skills from her previous Pokemon training in new and unexpected ways in the Alola region. Maybe she uses her knowledge of battling to help train local Pokemon that are having trouble evolving or learning new moves. This could also lead to her becoming a sort of mentor to younger trainers in Alola, passing on her wisdom and experiences from her travels in other regions.
One great Ash and Serena fanfiction in the Pokemon Sun and Moon setting is 'A New Adventure Together'. It has a really engaging plot where Ash and Serena meet up again in the Alola region and go on a journey to discover new Pokemon. The author does a wonderful job of capturing their characters and the unique charm of the Alola region.
Well, perhaps Serena could get involved in a mystery regarding a rare and powerful Pokemon in Sun and Moon. She could team up with some local trainers to solve it. Maybe there's a hidden cave where this Pokemon is rumored to be, and along the way, she discovers new forms of Pokemon she's never seen before. This would add an element of adventure to the fanfiction.
One interesting storyline could be an OC who is a new trainer in the Alola region. They start with a unique starter pokemon not typically found in the official games and their journey is about proving that this pokemon can be as strong as the traditional starters. For example, an OC with a Rockruff that has a special ability. They face challenges like the Island Trials in ways that are different from the main game characters, making new friends and rivals along the way.
One interesting storyline could be about a trainer who discovers a rare and powerful Pokemon that has been lost for centuries in the Alola region. This Pokemon could have unique abilities that change the way battles are fought in the fanfiction.
Ash is a great character to pair Serena with. Their history from previous adventures adds depth to the story. They could have new adventures together in Alola, with Ash showing Serena the ropes of the Alolan Pokemon world and Serena sharing her own unique skills. Another good option is Lillie. Serena and Lillie could bond over their love for Pokemon. They could help each other grow, like Serena teaching Lillie more about Pokemon battles and Lillie introducing Serena to the unique Alolan Pokemon.
Well, one possible plot could be Serena returning to Alola to participate in a grand Pokemon tournament. She might bring along some new Pokemon she's trained in her time away. Maybe she teams up with the main character from Sun and Moon to take on the toughest opponents.
The best aspect is seeing their relationship grow. In the Pokemon Sun and Moon setting, they get to experience new things together. For example, Serena can learn about the unique Alola Pokemon and Ash can show her around. Their interactions are really sweet, like when Ash introduces Serena to his new Alola friends. It adds a new dimension to their relationship.
One interesting storyline could be Serena and Kaa having an adventure in a mysterious forest. Maybe Kaa uses its abilities to help Serena find rare Pokemon. Another could be them teaming up to face a powerful villain who is threatening the Pokemon world. Serena could use her skills as a trainer and Kaa its unique powers.
One possible storyline could be Lillie and Moon teaming up to discover a new and rare Pokemon species in the Alola region. They might face challenges like rough terrains and other trainers who also want to find this Pokemon first.
One interesting storyline could be Ash's adventures in the Alola region with new Pokemon friends. He might face unique challenges and battles that are different from other regions. For example, dealing with the Totem Pokemon.