In many Pokemon fanfictions where Ash and Serena are injured, it could be due to a tough battle against a powerful opponent. Maybe they underestimated a new and dangerous Pokemon they encountered during their journey. Their injuries might then lead to them relying on each other more for support, both physically and emotionally. It could also be a chance for other characters to step in and help, like their Pokemon taking on extra caretaking roles.
One popular 'Pokemon Ash Injured' fanfiction might be where Ash gets injured during a battle with a powerful Legendary Pokemon. In these stories, often his friends like Pikachu and Misty have to step up to protect him while he recovers.
A common plotline is that they get injured while exploring a mysterious new region. Maybe they encounter a rare and powerful Pokemon that attacks them. Another is during a big competition where there's some sort of sabotage. For example, someone could have set a trap for them in the arena. And sometimes it's just a random accident like a landslide while they're on a mountain path.
One possible fanfic could be that Ash and his team are in a tough battle against a powerful opponent. Charizard, being overconfident, takes on a powerful attack head - on to protect Ash and the rest of the team. As a result, it gets seriously injured. Ash is devastated and has to quickly come up with a plan to both tend to Charizard's wounds and continue the battle. He uses his knowledge of Pokemon healing techniques he's learned over the years to stabilize Charizard and then makes a strategic retreat to a nearby Pokemon center.
One common plot could be Ash getting injured during a tough Pokemon battle. For example, when facing a powerful and rare Pokemon like Mewtwo. His Pikachu might try its best to protect him but Ash still gets hurt. Another plot could involve Ash getting injured while exploring a new and dangerous area in the Pokemon world, like an uncharted forest filled with wild and aggressive Pokemon. Maybe he stumbles upon a hidden lair of a group of Team Rocket - like villains who attack him.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
Misty would also be there to help. She's tough and resourceful. She might use her water - type Pokemon to cool down any burns if Ash has them, or to create a barricade against any approaching danger. Professor Oak could also play a role. He has a lot of knowledge and resources. He might send over some special medicine or a flying - type Pokemon to quickly transport Ash to his lab for proper treatment.
Maybe in the fanfic, Ash and his friends are exploring an ancient ruin. Charizard, being curious, wanders off and accidentally triggers a booby - trap. It gets severely injured. Ash is then on a race against time. He needs to find a way to heal Charizard before it's too late. Along the way, he meets a mysterious old trainer who gives him some advice on a rare and powerful healing method. But the method requires Ash to face his own inner demons and prove his worth as a trainer.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.
The impact of Ash's loss in the fanfiction could extend beyond just himself. His Pokémon might also feel demoralized. However, this could also be an opportunity for them to grow stronger together. Ash could start training more intensively, seek new training methods, or even try to catch new Pokémon to strengthen his team.