Well, another place to look could be Reddit. There are some Pokemon - related subreddits where fans might share or recommend Team Magma Courtney fanfictions. You could also ask in those communities if you can't find any through a simple search.
One interesting fanfiction could be about a trainer who tries to stop the conflict between Team Magma and Team Aqua. They might use their unique Pokemon team to negotiate peace. For example, having a water - type Pokemon that can communicate with Team Aqua's ideals and a fire - type that can relate to Team Magma.
Maxie is definitely one of the most iconic. As the leader of Team Magma, he drives the team's goals and actions. His strong beliefs in expanding the land for the benefit of humans and Pokemon make him a central figure in any Team Magma fanfiction.
First, know the lore of both teams well. Team Magma wants to expand the land while Team Aqua wants to expand the sea. Build your story around this fundamental conflict. You can start with creating an interesting protagonist, like a trainer who has a connection to both teams.
The plot might involve the Pokemon helping the grunt break free from Team Magma. You know, the grunt realizes that the Pokemon is more important to him than the team's goals. It could also be a story full of adventure as they try to escape the clutches of Team Magma while also developing their relationship. There could be other Pokemon and trainers involved, some trying to help them and some trying to stop them.
One plot idea could be a story where a young trainer gets caught up in Team Magma's plans to expand the landmass. They initially believe in Team Magma's cause but then discover the negative impacts it would have on the Pokemon ecosystem. As they learn more, they try to stop Team Magma from within.
Archie from Team Aqua is often quite popular. His strong leadership and his extreme views on expanding the ocean make him a complex and interesting character in fanfiction. Writers can explore his motives and what led him to form Team Aqua.
One interesting story could be about a Trainer who gets caught in the middle of a conflict between Team Aqua and Team Magma. Maybe they have to stop both teams from awakening a powerful legendary Pokemon that could destroy the balance of nature. This Trainer uses their unique bond with their own Pokemon to outwit the two teams.