One common theme is sacrifice. Ash might get seriously hurt while sacrificing himself to protect Serena from a powerful Pokémon or a villain. For example, he could jump in front of an attack meant for Serena.
One popular place to find such fanfiction is Archive of Our Own (AO3). Just search for 'Ash hurt protects Serena' there, and you'll likely get a bunch of stories. Some might focus on Ash getting injured during a Pokémon battle and then protecting Serena from the ensuing chaos.
There are many out there. Some focus on specific scenarios like when they encounter a powerful and dangerous Pokemon. In these fanfictions, Ash steps up, using his Pokemon and his own courage to ensure Serena's safety. For example, in a fanfic set in a dark forest, Ash senses danger before Serena does and immediately goes into protection mode.
The appeal lies in the contrast between Ash's usually adventurous and goal - oriented self and his soft side when it comes to protecting Serena. It also gives fans a chance to explore different dangerous situations and how Ash would react to keep Serena safe. In many of these fanfictions, we see Ash's growth as a character in terms of his ability to care for someone deeply.
Ash would be very worried. He would immediately rush to Serena's side. He might stop whatever he was doing, like a Pokemon battle or exploring, and focus all his attention on her. He would probably ask if she's okay and check for any serious injuries.
You can try some popular fanfiction websites such as FanFiction.net or Archive of Our Own. Just use the search bar and type in 'pokemon ash and serena hurt' to see what comes up.
One possible 'pokemon ash and serena hurt fanfiction' could be about Ash getting injured during a battle and Serena trying her best to take care of him. Maybe he's poisoned by a wild pokemon's attack and Serena has to find the antidote in a hurry.
In fanfiction, anything can happen. However, without specific details of the particular fanfic, it's hard to say for sure if Serena is pregnant. Some fanfictions might explore this plotline for the sake of drama or character development. It could add an interesting twist to the relationship between Ash, Serena and Lemon, but it also depends on the overall theme and direction the author wants to take the story.
They would probably be a bit shy at first. Ash being his usual energetic self but also respectful of Serena's space in the tent. Serena might be a bit nervous but also excited to be sharing such a close space with Ash.
A great idea could be a fanfiction about Ash and Serena traveling together to a new region to discover rare Pokémon. They could face challenges, make new friends, and help each other grow as trainers. This would focus on their relationship in the context of the Pokémon world and their shared goals.