Definitely Beetlejuice. His character is so complex and full of contradictions. He can be both menacing and comical at the same time. However, the other dead souls in the musical can also be interesting. They each have their own stories and quirks. For instance, those dead football players could have their own mini - adventures in a fanfic. And of course, Lydia's father, Charles, can be an interesting character to focus on. He's a bit uptight and traditional, and seeing how he copes with the Beetlejuice - related chaos in a fanfic can be quite engaging.
I think Beetlejuice is the top choice. He has such a unique personality, full of mischief and a desire for chaos. But also, Delia, with her new - age beliefs and her somewhat out - of - touch nature, can be really fun to explore in fanfic. You can put her in all sorts of strange situations in the Beetlejuice world and see how she reacts. Additionally, the sandworm could be an interesting focus. It's this huge, menacing creature, but there could be more to it, like a hidden gentle side or a backstory about how it came to be in the underworld.