One interesting fanfiction could be where Lance and Krolia team up on a secret mission. They might discover hidden powers within themselves during the process. Another could be a more emotional one where Lance helps Krolia deal with her past traumas.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you can easily search for Lance meets Krolia ones using the search bar.
The protagonist of Pokemon Yellow is an obvious potential ally. They are on their own journey to become a Pokemon master and could easily team up with Lance. Another could be Professor Oak. He has a vast knowledge of Pokemon and could provide valuable information and resources to Lance. Also, the Gym Leaders in the Pokemon Yellow world could be allies. They are all skilled trainers in their own right and could join forces with Lance when facing a common threat, like an evil team trying to disrupt the Pokemon League.
They might have all sorts of adventures. Maybe they go on a family fishing trip and Keith has to learn how to bait a hook, which he's never done before. And Lance's family could be showing Keith all the local hidden gems in their town.
There could be his teammates. They might play a role in either helping Lance face his dark sides or accidentally making them worse. For instance, if one of his teammates is really competitive, it could add to Lance's insecurities.
One common theme is physical injury and recovery. For example, Lance might get seriously injured in a fight and then have to go through a long process of healing, which shows his strength and determination.
I'm not entirely sure as I haven't specifically read that fanfiction. But generally, it could be about a character named Ash who is trained by someone named Lance. It might involve Ash learning new skills, going on adventures, or facing challenges under Lance's guidance.
The main characters are likely Ash and Lance. Ash is a very popular character from the Pokémon series, and in this fanfiction, Lance is the one training him.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of Pokémon - related fanfictions, including many about Ash and Lance.
One great Ash and Lance fanfiction could be 'The Adventure Beyond the League' where they team up to face a new, powerful threat. It has a really engaging plot.