A common theme is the power struggle. Castiel is often seen as a more 'pure' angel, while Lucifer is the rebellious one. Their interactions are filled with this underlying battle for dominance or influence. Another theme is redemption. Sometimes Lucifer is portrayed as having a chance at redemption, with Castiel being a key factor in that.
A common theme in such fanfiction is the struggle between Castiel's sense of duty and Lucifer's chaos. Castiel is often seen as a dutiful angel trying to do what's right, while Lucifer represents the breakdown of order. Fanfictions may show Castiel being conflicted when dealing with Lucifer, as he may see some of his own doubts about Heaven's ways reflected in Lucifer's actions.
Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
In the world of 'lucifer meets lucifer fanfiction', a common plot could involve a clash of egos. When two such powerful and complex characters meet, there's bound to be conflict. Some stories may also explore a romantic or at least a deeply emotional connection between them, as they are essentially the same being in different forms, and this connection can be both fascinating and disturbing at the same time.
In some fanfictions, the relationship between Castiel and Lucifer is depicted as a complex rivalry. They both have strong connections to the Winchesters, but for different reasons. Castiel wants to protect them, while Lucifer has his own dark plans. This creates a lot of tension and interesting plotlines where the two angels' paths cross and they engage in battles of wits and power.
One popular 'castiel lucifer fanfic' is 'The Fallen and the Faithful'. It delves into their complex relationship and how they interact in an alternate universe. Another great one could be 'In the Shadows of Heaven', which explores their power dynamics and the moral gray areas between them.
I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.
One idea could be that Alec, from the Malec universe, stumbles into Lucifer's realm while on a mission. He's initially confused and on guard. Lucifer, being the charming devil he is, tries to make a deal with Alec. But Alec, being true to his Shadowhunter values, refuses. This could lead to an interesting dynamic where Lucifer is intrigued by Alec's integrity and Alec is both repelled and fascinated by Lucifer's world.
In a 'Castiel meets the Avengers' fanfiction, there could be a lot of interesting scenarios. Maybe Castiel tries to understand the Avengers' concept of heroism which is very different from his own in the Supernatural universe. He might be intrigued by their technology and try to figure out how it works in relation to the magic he is used to.
Well, in the fanfiction, Castiel might be initially confused by the Ghostfacers' methods. He could be intrigued by their enthusiasm despite their somewhat amateurish approach to the supernatural. Maybe he'd end up helping them out in a case without really meaning to at first, like when they misinterpret a sign and Castiel corrects them, leading to a successful hunt.