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.
Serena could be a bit shy at first. She might blush deeply and look down for a moment. But then, she would look into Ash's eyes with love and admiration. She would tell Ash how much she loves him and how she has been hoping for this moment. She would accept his proposal gracefully, and they would share a tender embrace while their Pokémon friends cheer around them.
Serena could be a fictional story crafted by the author's imagination. But sometimes, fictional works can touch on universal themes and emotions that feel true to many people's experiences.
There are several characters that would work well. Lillie from the Alola region could be a great addition. She has her own unique story and relationship with Pokemon that could contrast with Serena's. Another option is Kiawe. His fiery personality could create some interesting dynamics with Serena. Then there's Gladion, who could bring an element of mystery and a different approach to Pokemon battles. And of course, Professor Oak. His wisdom and knowledge of Pokemon could be a valuable asset in a Serena Pokemon fanfic. He could be the one who sends Serena on a special mission or gives her some crucial information about a rare Pokemon.
A rival character would be very likely. This could be someone who believes they are more worthy of the throne. They might try to undermine Serena at every turn, creating a lot of conflict in the story.
Definitely Ash, as he's the center of Serena's affections. But also, there could be new female characters created specifically for the fanfiction. These new characters might be trainers who are really good at Pokemon battles and catch Ash's attention. Serena would then become jealous of their skills and the attention Ash gives them. Additionally, any of Ash's Pokemon that seem to have a special connection with another character could also be part of the scenario that makes Serena jealous. For example, if Pikachu seems to like a new girl more than Serena, that could be a reason for her jealousy.
One possible plot development could be that Ash and Serena have to deal with the new responsibilities of having a baby. This might cause some tension between them as they figure out how to balance their own goals and the needs of the family. Lemon could be involved as a support system, or perhaps as a source of conflict if there are romantic feelings involved among the three. For example, if Lemon has feelings for Ash, Serena's pregnancy could make those feelings more complicated.