Their relationship could be characterized as a clash of the old and the new. Lucifer represents the ancient, rebellious celestial being, while Adam is the new creation. It could be a relationship full of tension as they come from such different worlds.
One interesting plot idea could be that Lucifer, seeing Adam's pure and innocent nature, decides to test him in various ways. He might create illusions of worldly temptations to see if Adam can resist. Another could be that Adam, being curious about the fallen angel, tries to understand Lucifer's fall from grace and in the process, they form an unlikely friendship that challenges the norms of heaven and earth.
One possible plot twist could be that Adam has a hidden weakness that Sun discovers during the fight. Maybe it's related to his past or his weapon. Another could be that Sun is actually being controlled by an outside force and Adam has to decide whether to take advantage of it or help him.
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.
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.