One interesting storyline could be about the benders facing a new and powerful supernatural entity that challenges their abilities in unexpected ways. For example, they might encounter a spirit that can manipulate time, causing chaos in their world.
Maybe there's an antagonist, like a dark bender who has turned to the wrong side and uses their powers for evil. This character would create conflict for the main bender characters, constantly challenging them and trying to disrupt the balance in their world.
You can start by looking on fanfiction.net. It has a large collection of 'Supernatural' fanfictions, including those related to 'The Benders' episode. Another great place is Archive of Our Own (AO3). There are many talented writers who post their works there, and you can easily search for 'Supernatural The Benders' fanfics using the search bar.
War and its consequences are likely to be a main theme. It may also explore the idea of power struggles among different groups.
One common theme is power struggle. Abaddon is a powerful demon, so many fanfictions explore her battles for power, either against other demons or the Winchesters. Another theme is her backstory. Writers often delve into her past to give more depth to her character.
One common theme is forbidden love. Maybe a vampire falling in love with a werewolf in the academy, which goes against the norms of their species. Another is the underdog rising to power. A student who is initially underestimated but then discovers some great power within themselves.
One common theme is redemption. Since Adam had a rather rough deal in the show, a lot of fanfictions explore his journey towards redemption. Another is family. How he fits into the Winchester family dynamic, whether it's forming a bond with Sam and Dean or finding his place in the larger family of hunters.
One common theme is redemption. Cassiel has made some mistakes in the show, and in fanfictions, writers often explore his journey to redeem himself. For example, he might try to make up for his actions by helping Sam and Dean in a more selfless way.
One common theme is the aftermath of the events in the episode. Writers often explore how the characters deal with the consequences. Another theme could be the relationships between the characters, like Dean and Sam's bond, which might be further developed or tested in the fanfictions.
One common theme is redemption. Ketch has done some not - so - great things in the show, and in fanfiction, he often gets a chance to redeem himself. For example, he might help the Winchesters in a big way to make up for his past actions.