Castiel's relationship with his powers might also change. He may have to limit his use of angelic powers to protect the unborn entity. This can cause internal conflict as he is used to using his powers freely. At the same time, it can also show his growth as a character, as he is willing to sacrifice some of his power for the sake of something new and precious.
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.
If we're talking about Castiel from 'Supernatural', his story is complex. He is an angel who rebels against Heaven's orders. He becomes involved with the Winchester brothers, Sam and Dean, and helps them fight various supernatural threats. He has his own journey of self - discovery, dealing with his loyalty to Heaven, his newfound friendships with humans, and the struggle between good and evil in the context of the show's universe.
Harry Potter himself would be very interesting. Castiel could be intrigued by Harry's connection to Voldemort and his role as the 'Chosen One'. He might offer Harry some unique insights from his angelic knowledge about dealing with dark forces, which could be different from what Dumbledore has taught him.
In dean - castiel relationship in such fanfiction, there's often a deep emotional connection. They might show a side of extreme vulnerability towards each other. Their relationship can be intense, with a lot of passion and a sense of 'destined to be together' kind of feeling.
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.
Lucifer can be a good comic depending on your preferences. If you enjoy dark and mythical themes, it's likely to appeal to you. The character development is strong, and the world-building is immersive.