One common plotline could be Riolu's transformation being triggered by a powerful emotion like protecting its trainer. For example, in some fanfictions, when the trainer is in great danger, Riolu's bond with the trainer causes it to transform into a more powerful form, perhaps Lucario. This showcases the deep connection between Pokemon and trainer.
A third common plotline could be Riolu's transformation as part of a prophecy. In the fanfiction world, there could be a prophecy that a Riolu with a certain mark or trait will transform and bring balance to the Pokemon world. This would involve Riolu going on a journey to fulfill this prophecy, meeting various allies and foes along the way, and growing both in strength and character during its transformation process.
Another plotline might involve Riolu's transformation due to a special item or a mysterious place. Maybe it discovers an ancient stone that unlocks its latent powers and transforms it. This could lead to Riolu having to learn to control its new abilities while also facing challenges from other Pokemon or trainers who are interested in this new powerful form.
Riolu may become more confident. As it transforms, it realizes it has new powers and abilities. For instance, if it gains the ability to sense auras more strongly, it becomes more self - assured in battles and interactions with other Pokemon.
In some fanfictions, there's a plotline where Riolu is initially distrustful of humans because of some past trauma. Ash, with his kind heart, slowly breaks through Riolu's barriers. They then train together to enter a big Pokemon tournament. Riolu's unique fighting style, combined with Ash's battle strategies, makes them a formidable team in the tournament.
One possible fanfiction could be about a Riolu who discovers a hidden power within the mystery dungeons. It might start with Riolu being separated from its group and having to rely on its own wits and newfound abilities to survive and find its way back. Along the way, it could meet various other Pokemon and form unexpected alliances.
Riolu's personality could develop in a way that it becomes more confident. At first, it might be scared of battles and new situations. But as Ash encourages it and they have small victories, Riolu starts to believe in itself. For example, after winning a small battle against a weak pokemon, Riolu realizes it has the potential to be strong.
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.
In these fanfictions, Ash usually has a deep sense of empathy towards Riolu. He can sense when Riolu is sad or excited. For example, if Riolu is homesick, Ash will do his best to make Riolu feel at home. He might find Riolu's favorite foods or create a comfortable place for it to sleep. His caring nature towards Riolu is a big characteristic in these stories.
One common theme is identity. When a character transforms into an anthro pokemon, they often struggle with who they are now. Are they still the same person or something new? Another theme is acceptance. They have to deal with how other pokemon and humans in the story accept or reject their new form.
The theme of friendship often pops up. A human - turned - Pokemon might make new Pokemon friends and learn about loyalty and cooperation. Also, there's the theme of finding a way back. Most of these fanfictions involve the character trying to reverse the transformation at some point. They might search for a special item or a powerful Pokemon that can help them.
Often, in reborn as a Pokemon fanfiction, the protagonist might have some special ability. This could be the ability to understand Pokemon language perfectly. They then use this to their advantage, making strong bonds with their Pokemon team. Also, there are plotlines where the character has to save the Pokemon world from an evil organization that wants to use Pokemon for bad purposes.
Riolu could find a rare treasure in the mystery dungeons.