It's not suitable. Fanfiction should be a form of creative expression that respects the original work and its fans. Gay sex fanfic in the context of 'hunterxhunter' may be offensive to some viewers or readers. It's better to focus on more positive and inclusive themes that are in line with the spirit of the anime, like friendship, adventure and character growth.
Friendship is a wonderful theme. You could write about the deepening of the bonds between the main characters. Like how Gon and Killua's friendship evolves as they face more challenges together in the Hunter world. It could involve them having heart - to - heart conversations, supporting each other in battles and learning more about each other's pasts.
The 'Man of War' in Malibu Comics often has superhuman strength and advanced combat skills. It also has a complex backstory and a mission to protect justice.
The choice of background music for cartoons really varies. If it's a children's cartoon, maybe light and playful tunes would be nice. For more serious or dramatic cartoons, orchestral or emotional music could fit the bill. Also, the tempo and rhythm should match the action on the screen.
Tentacle hole manga in its raw form often features explicit and unconventional imagery. It might have bold and unfiltered depictions that can be quite shocking for some.
It could be quite an unusual and perhaps somewhat intimidating image. Maybe it shows Ronaldo's features combined with the menacing look of a cobra head.