Well, there could be a fanfic where Lucifer forms an unexpected alliance with an exorcist. This alliance could be born out of a mutual understanding of a greater threat that is looming over both the demon and human worlds. They have to put aside their differences and work together, which makes for a really exciting story.
You can try fanfiction.net. It has a large collection of fanfics from various fandoms, including 'ao no exorcist', so there's a good chance of finding some great lucifer - related fanfics there.
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.
He wasn't sure. The information I know about the source and background of the Ao Ao awaiting arrest comic is very limited. It could be adapted from a novel or an original. If you can provide more information about the manga, I will try my best to answer your questions.
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.
There were several possibilities for replying to Ao. First, replying to a question might indicate that you agree with what the other person said, and that you think what the other person said makes sense. The other possibility was that he was just perfunctory, indicating that he was not interested in the conversation or did not want to continue. To be specific, the reply might be because the other party was busy and felt that chatting with you was not important, so they had to wait. It might also be because the other party's response was slow and did not deliberately brush you off, but they did not reply in time. It might also be that the other party was indeed brushing you off. According to the specific context and the other party's performance, he could more accurately judge the meaning of the reply.
I don't really know who ao chan is just from this statement. It could be a character from a specific story, game or anime that the speaker is familiar with.
I'm not entirely sure specifically what 'lucifer fiction' refers to. It could potentially be a genre of fictional works that somehow involve the concept of Lucifer, perhaps in a religious, mythological, or even a more creative and fictionalized context.