Great you want to make a Monkey Island fangame! However, you're not going to accomplish this on your own. Also, you write about the music and art style but do you already have an idea how you are going to make the actual game?
As for the story: it is a nice idea to write a story about the secondary characters, but in this case I don't think it will work. Guybrush Threepwood as a protagonist is so important for the MI series that you can't leave him out of it. So if you don't use Guybrush I suggest you think about a story that has a setting for the game that is quite close to MI (something with pirates
) but not with the actual MI (secondary) characters.
Of course I want to help, but first I think you'll need a more solid idea.