The story could revolve around the unique abilities or personality of the shiny Riolu and how it helps Ash stand out among other trainers. Maybe it attracts unwanted attention or opens up unexpected opportunities for them.
The fanfiction might focus on how this discovery changes Ash's plans and strategies for his journey.
It could potentially lead to Ash forming a stronger bond with Riolu earlier on and impact their growth together. Also, it might introduce new challenges or allies because of the shiny Riolu's presence.