I agree with Farlander. Elaine is portrayed as a character that is often 10 steps ahead of everyone else, and I don't think that would translate over very well if she went from NPC to player character.
Could she have a larger role in a future Monkey Island? Sure, I don't see why not. It would be nice to see her character further developed, but I just can't see anyone being the main character of a Monkey Island game aside from Guybrush.