Castiel can also be an interesting character to pair with Chuck. Castiel's loyalty to the Winchesters and his own angelic perspective can create a contrast with Chuck. Their interactions could explore themes of faith and free will, especially considering Chuck's role as the 'creator' of their world.
In some Supernatural Chuck fanfictions, Chuck is often portrayed in different lights. For example, there are stories where he tries to rewrite the entire Supernatural story in a more positive way. Some fanfictions explore his relationships with the other characters more deeply. There are also those that put him in situations where he has to learn from the humans he created, which is a really fascinating concept as it flips the traditional creator - creation relationship on its head.
The character of Chuck in Supernatural is very complex. Fanfiction about him allows fans to explore different aspects of his personality that weren't fully explored in the show. For example, his motives and his true feelings.
One common theme is the exploration of power. Since Chuck is a powerful being in the Supernatural series, fanfictions often deal with how Sam interacts with Chuck's power and what it means for their relationship.
Another storyline could be that Chuck is not as all - knowing as he seemed. Fan fictions sometimes explore the idea that there are things beyond his control or knowledge in the Supernatural universe. He then has to rely on Sam and Dean's instincts and skills more than he expected, leading to a more equal partnership in their adventures.
Another great one is 'Sam and Chuck: An Unexpected Alliance'. It shows how Sam and Chuck, who seem to have different agendas in the show, come together and form an alliance that changes the course of events in the Supernatural world.
Nate could be a great addition. He could be a confidant to either Chuck or Blair, and his own storylines could intersect with their married life. For example, if he gets into a business deal that affects Chuck and Blair's business.