In 'supernatural is real fanfiction', angels and demons are usually very interesting characters. Angels with their celestial powers and complex hierarchies, and demons with their devious natures. Their interactions with humans and with each other can create a lot of drama and tension. Another interesting type of character could be the seers or prophets who can foresee events related to the supernatural, and their struggle to either change or accept what they see.
Oliver Queen from Arrow and Dean Winchester from Supernatural make an interesting pair. They are both strong - willed leaders. Oliver's combat skills and Dean's hunting skills could complement each other well. They could go on adventures together, fighting both the criminals in Star City and the supernatural creatures in the Supernatural world.
Sam Winchester is definitely one of the most interesting. His dual life at Stanford and his hunting heritage make him complex. Also, Jessica, his girlfriend at Stanford, as she was an important part of his 'normal' life and her death was a turning point.
The Bell Witch Haunting is quite interesting. It was a series of events that happened to the Bell family in Tennessee. There were strange noises, physical attacks on family members, and the witch was said to communicate with them, often in a menacing way. It was so well - known that it became part of local folklore.
Definitely Castiel. Castiel has his own complex relationship with death and the divine in the Supernatural world. Pairing him with the Angel of Death would create a really interesting dynamic. They both have different views on mortality and the afterlife, which could lead to some great character interactions.
Sam Winchester would be really interesting. His research skills could help figure out the strange rules and secrets of the Hunger Games world. Peeta Mellark could also be a great addition. He's good at strategy and forming alliances, and he could form an alliance with Sam. And then there's Crowley. The King of Hell could be a wild card in the Games, either helping or hindering the tributes depending on his own motives.
Castiel is also a favorite. His angelic nature and his complex relationship with the Winchesters can take on new dimensions in inflation fanfiction. For instance, if there's an inflation of angelic powers, how he navigates that while still being involved with Sam and Dean makes for great story material.
The witches are often popular. They have a lot of powers and can be both good and bad, which gives a lot of room for character development. For example, a witch might start off using her powers for selfish reasons but then learn to use them for the good of the community.
Sometimes, there are recurring characters like certain spirits or demons that the Ghostbusters keep encountering. These characters can be considered main in the sense that they drive the plot forward by constantly challenging the Ghostbusters and forcing them to come up with new strategies to deal with the supernatural.
Natsu would be one of the most interesting. His fiery nature and his constant search for adventure would make his reactions to the real world really exciting. He might go around trying to find things that can give him the same thrill as fighting a powerful magic - using enemy in his world.
Arya Stark and Castiel. Arya is a skilled assassin with a thirst for revenge. Castiel is an angel who often struggles with understanding human emotions. Their interactions could be really fascinating. Castiel could help Arya understand the deeper spiritual side of her revenge, while Arya could teach Castiel about the importance of personal vendettas in the context of the cut - throat world of Westeros.