One common theme is friendship - turned - love. Usually, they start as friends in the fanfic and then slowly realize their deeper feelings. For example, they might support each other through school problems first and then those feelings transform.
There's a fanfic titled 'Riko and Maki's Adventure in Love'. In this story, they go on a journey together and along the way, they face various challenges that strengthen their bond. The author does a wonderful job of showing the emotional growth of Riko and Maki as they navigate their feelings for each other.
One common theme is the exploration of their relationship development. It could be about how they gradually fall in love or how they overcome obstacles in their relationship.
I'm not sure of specific 'love lab riko and satoshi fanfiction' off the top of my head. But you could also try joining relevant fan groups on social media platforms. Fans in those groups might be able to recommend some great fanfictions. They may even share some of their own works that are not widely known on the big fanfiction websites.
One great fanfic could be 'Reg and Riko's Adventure Beyond'. It beautifully captures their relationship as they explore deeper into the Abyss. The author does an amazing job of showing Reg's protective nature towards Riko.
Another theme is the exploration of their different personalities. Nico's energetic and sometimes bossy nature combined with Maki's more reserved and wealthy - girl - with - a - secret - passion personality can create interesting story dynamics. For example, a fanfic might focus on how Nico tries to break through Maki's reserved shell.
The theme of overcoming obstacles is also quite common. It could be things like dealing with family expectations, or differences in their future plans. In many fanfics, they have to work through these issues to be together. And of course, the theme of the power of love, how their love for each other helps them face any difficulties that come their way.
One common theme is friendship. Many fanfics focus on how they become friends and the things they do together. For example, going to school events or just hanging out.
Some smaller, more niche fanfic communities or forums dedicated to the source material of Kanna and Riko might also have unique 'kanna x riko fanfic'. You can search for these by looking for forums related to the series that these characters are from. However, make sure to follow the rules of these communities when looking for fanfics.