Inosuke is interesting as well. He was raised by wild boars and has a very wild and unruly nature. He is extremely competitive and has his own strange ways of fighting demons. His rough exterior but good heart make him a memorable character.
Zenitsu is quite interesting. His cowardly nature at first, which is in contrast to his amazing lightning - based fighting skills when he's unconscious, creates a lot of comical and engaging moments in the light novel. Inosuke is also fascinating with his wild and boar - headed appearance and his brute - force fighting style.
One popular trope is the 'power - up' trope. In many fanfictions, characters from both Fairy Tail and Demon Slayer gain new powers or improve their existing ones in unique ways. For example, a fanfiction might have Tanjiro learning some Fairy Tail magic to enhance his demon - slaying abilities. Another common trope is the 'team - up' trope. Fans love to see characters like Natsu and Tanjiro working together against a common foe. It creates a lot of exciting action scenes and interesting character dynamics.
In Naruto fanfiction, a popular theme is power - ups. Fans like to write about Naruto or other characters getting new and stronger abilities. For Demon Slayer, the theme of redemption for demons is quite common. In Fairy Tail, friendship and the power of the guild are often - explored themes. For instance, many Fairy Tail fanfictions show how the strong bond between guild members helps them overcome difficulties.
A popular theme is the power - up theme. Lucy might gain new powers related to Demon Slayer abilities in the fanfiction. For instance, she could develop a way to use her magic to purify demons like the Demon Slayers do with their swords.
Naruto himself is often a popular character in these fanfictions. Writers like to put him in situations where he teams up with the Phoenix Slayer. Another popular character could be Sasuke. His brooding nature and powerful abilities make for interesting interactions with the Phoenix Slayer concept. Also, Kakashi is often included as he is a fan - favorite and his wisdom can add depth to the story when dealing with the new element of the Phoenix Slayer.
Well, Naruto is no doubt a top choice. He has a huge fan base already, and in Dark Slayer style fanfiction, fans are interested in seeing how he would handle the new and dangerous dark powers. Sasuke is another one. He has always been on the edge between light and dark, so it's natural for him to be a central figure in these stories. Then there's Orochimaru. His obsession with power and his dark experiments make him a very fitting character for Naruto Dark Slayer style fanfiction. He can be either the main villain or a source of the dark powers that others are after.
Buffy herself is quite popular. Fans like to see how she'd interact with the Twilight characters. Edward also often features prominently as his brooding nature contrasts with Buffy's more action - oriented personality. Another popular character is Spike from Buffy. His edgy vampire persona can create interesting dynamics when paired with the Twilight vampires.
Obviously, Naruto himself is very popular as he is the main character of the original Naruto series. The White Dragon Slayer is also a central figure in these fanfictions. Sakura might also be a popular character as she could interact with the new elements brought in by the White Dragon Slayer. Kakashi could be involved too, perhaps mentoring the White Dragon Slayer in some ninja skills.