Help! Any tips for a Batman fan fiction?

I've been thinking about a Batman fan fiction for sooo long but I haven't yet started because I'm not sure where to begin from...
I thought that I'd do a fan fiction that would tell the story of the Dark Knight BEHIND the scenes. That means the parts where the Joker is planning his next move or when Batman is on his normal night shift. And maybe perspectives from different people than the people that were filmed in the scene, like what Joker was doing when Two Face was killing the people responsible for Rachel's death.

And yes, I would probably mostly write about Joker's perspective :D

I know this would be a very long and hard writing experience for me since I haven't got much time to write and I'm so lazy. I thought I could start writing this summer.

Any ideas from what perspective I should start from?