It's difficult to say exactly what the plotlines would be as it's such an unusual concept for Pokemon fanfiction. But it could be that some sort of time - travel or dimension - hopping event leads to a world where different biological rules apply to Ash and Gary, resulting in pregnancy. It could also be a story about them having to deal with the social and personal implications of such an unexpected situation within the Pokemon world.
Maybe a plotline could involve a powerful Pokemon's ability that has unexpected consequences and results in Ash or Gary becoming pregnant. Or perhaps it could be part of a very elaborate and strange fan - created prophecy that they must fulfill by experiencing this situation. However, these types of plotlines are not typical in the Pokemon fanfiction genre and are more on the extreme and creative side.
One possible plotline could be that it's set in an alternate universe where some magic or strange phenomenon causes them to become pregnant. Another could be a scientific experiment gone wrong scenario. But these are just wild speculations as such a concept is very far - fetched from the original Pokemon concept.
One possible theme is the transformation of a rivalry into something deeper. Gary and Ash were rivals, but in this fanfiction, it could be about how that rivalry can turn into love or a very strong friendship. Another theme might be self - discovery. Gary might be on a journey of realizing his true feelings for Ash.
I'm not sure if there is specifically an 'appropriate' one. Fanfiction can vary greatly in nature and content. You could try looking on well - known fanfiction websites like FanFiction.net or Archive of Our Own and search for it while filtering through the results to find something that adheres to your idea of appropriate.
One popular plotline is a friendly rivalry turned into a partnership. Ash and Gary, who have been competing for so long, realize they can achieve more together. They might team up to face a powerful new threat in the Pokemon world, like a mysterious and extremely strong Legendary Pokemon that is causing chaos. Another plotline could be a journey of self - discovery for both of them. They travel together and learn from each other's different training styles, which helps them to grow not only as Pokemon trainers but also as individuals. There are also fanfics where they might have to overcome a misunderstanding or an old grudge between their families that has been affecting their relationship all along.
A plotline could involve a curse or a strange experiment gone wrong that causes Ash to be in a situation where he has to give birth. Or perhaps there's a parallel universe where the rules of Pokemon are different and male characters can give birth. Ash could be on a journey in this universe and have to deal with this new and unexpected situation, learning about himself and the world around him in a completely different way.
One possible plotline is that Ash is on a journey to prove himself against the toughest opponents and finally faces Brandon. He uses his Pikachu in a unique way to gain an edge in the battle.
Maybe Harrison comes to Ash's town and challenges him. Ash accepts, and during the battle, there are flashbacks of their past experiences and how they got to where they are as trainers. This could add depth to the story and show their growth.
One plotline could be that Ash uses his ability to speak to Pokemon to solve a mystery. For instance, a group of Pokemon are acting strangely, and by talking to them, he discovers that there is a hidden force affecting them. Another could be that he has to convince a powerful and angry Pokemon not to destroy a village by communicating with it on a deeper level.
If such a fanfiction exists, a possible plotline could be Misty dealing with the unexpected pregnancy while still trying to pursue her Pokemon - training journey. She might have to balance her responsibilities as a trainer with the new - found situation of being pregnant. Another plot could involve the reactions of her friends, like Ash and Brock, and how they support or react to her pregnancy.
One possible plotline could be that Gary gets injured during a tough Pokemon battle. His rival, Ash, might then have to step in and take care of Gary's Pokemon temporarily. This could lead to a deeper understanding between them and a change in their competitive relationship.
One possible plotline could be that Ash somehow gets merged with Lugia's power during an unexpected encounter in a mysterious place. Maybe he's trying to protect Lugia from some evil force, and in the process, a magical transformation occurs. This could lead to him having to learn how to control Lugia - like powers, such as flying and using powerful water - based attacks. Another plotline could involve Ash being transformed into a Lugia - like form and having to hide it from his friends at first, fearing their reaction.