You can try looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash catches Riolu' in their search bars, and you should be able to find relevant stories.
In another plot, Riolu could be a Pokémon that has been abandoned by its previous trainer. Ash stumbles upon it while on his journey. At first, Riolu is very distrustful of humans, but Ash shows it kindness and patience. Through Ash's training methods and the bond he forms with his other Pokémon, Riolu slowly warms up to him and allows itself to be caught.
Adventure is also prevalent. Ash and Riolu go on various quests together, exploring new lands, encountering new Pokémon and trainers. These adventures not only test their skills but also strengthen their relationship as they face difficulties side by side.
It could be a fan - made story where Ash has a special connection or interest in Riolu. Maybe it involves Ash's adventures with a Riolu, like training it or going on quests together.
One idea could be that Ash has a strange encounter in a mysterious forest. There, he finds a Riolu in distress. As he tries to help the Riolu, a strange energy transfer occurs, and he starts to take on some of Riolu's characteristics. He might gain Riolu's heightened senses, like its ability to sense auras more acutely. This could lead to new adventures as he has to adapt to these changes while still trying to pursue his Pokémon journey goals.
A fanfiction could explore how Ash and Riolu's relationship develops over time. At first, Riolu might be a bit shy and reserved. But as they travel together, Riolu starts to open up more. Ash trains Riolu not just to be strong in battles, but also to be a good friend. They might have heart - to - heart conversations under the starry sky, sharing their dreams and fears, which is a really touching aspect of their journey.
The fanfiction could have a plot where Riolu has a hidden power that only Ash can unlock. As they face tougher opponents, this power gradually reveals itself. It could be a psychic - like ability that allows Riolu to communicate telepathically with Ash. This new development in their relationship changes the way they battle and interact with other Pokémon.
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.
Riolu could start off as a shy and scared Pokemon. But as it spends more time with Ash, it would become more confident. It might learn new moves and become more brave in battles, evolving into Lucario eventually as it gains more experience and trust in itself.
Well, it could be about Ash finding a lost Riolu in the forest. Ash decides to take care of Riolu and train it. They go on adventures together, facing different Pokemon battles. Riolu could grow stronger under Ash's guidance and they might aim to win a big Pokemon tournament.
Well, perhaps Ash could be magically transformed into a Riolu. He might then struggle to adapt to his new form. For example, he would have to learn to use Riolu's unique moves like Force Palm in a completely different way compared to using his Pokémon attacks as a human. He could also face challenges in communicating with his old Pokémon friends as a Riolu.