One interesting concept could be how Buffy's fighting skills are tested in the hellish environment. Since she's used to fighting demons on Earth, in Hell, the rules and the power levels of the foes might be completely different.
Perhaps the exploration of her relationships in a 'Buffy in Hell' fanfic is engaging. Her friendships, like with Willow and Xander, might take on new dimensions as they try to reach out to her or she tries to hold onto memories of them while in Hell.
One interesting concept could be Buffy's slayer powers combined with Batman's detective skills. For example, they could team up to solve a mystery that involves both the supernatural elements Buffy is used to and the criminal underworld that Batman patrols. Maybe a powerful vampire is working with a Gotham crime syndicate, and their combined expertise is needed to take them down.
One unique concept is time - travel fanfics. For example, some fics send Buffy back in time to the days when the first Slayer was alive. It changes the whole dynamic as she has to deal with the ancient vampire - slaying traditions and different versions of vampires.
Well, in some Percy Jackson Hell fanfiction, the idea of the gods having a different relationship with Hell is really cool. For example, perhaps they have to make deals with the rulers of Hell to save their demigod children. This can lead to some really complex power dynamics and moral dilemmas for the characters, as they have to navigate between the will of the gods and the rules of Hell.
One interesting concept could be Buffy characters having to adapt to the use of the Force in the Star Wars universe. For example, Buffy herself might struggle to understand the different rules of combat when faced with lightsabers and Force powers. Another concept could be the Jedi or Sith being introduced to the concept of slaying vampires and demons like in Buffy's world.
One interesting concept could be the exploration of impossible odds, just like the phrase'snowball's chance in hell' implies. The fanfiction might play with the idea of characters defying extreme improbability.
In such fanfictions, the power hierarchy might be an interesting aspect. If there are lords of Hell involved, how do they rank in power compared to the powerful wizards in the Harry Potter world? Voldemort was considered extremely powerful, but how would he fare against a lord of Hell? Some fanfictions might explore this by creating epic battles where the rules of magic are tested and new alliances are formed. Maybe Dumbledore tries to make a deal with a less - evil lord of Hell to gain an edge against Voldemort.
One interesting Buffy the Gamer fanfic could be about Buffy using her gaming skills to outwit a new type of demon. Maybe she has to complete a series of virtual challenges in a demon - created game world to save her friends.
There might be a fanfic where the Buffy Builder discovers a hidden power within himself that changes the course of the fight against evil. In this story, his character development is really interesting as he learns to control this new power.
One common theme is sisterly love and protection. Given their relationship in the Buffyverse, many fanfics explore how Buffy looks out for Dawn and vice versa in different situations.
One interesting Buffy dungeon fanfic could be 'The Dungeon of Shadows'. It has a great plot where Buffy has to face new and mysterious foes in a dungeon setting. Another might be 'Buffy in the Forgotten Dungeon' which focuses on her journey of self - discovery while trapped in an ancient dungeon.