With what we know of Elaine's character and how she interacts with other people when Guybrush isn't around, I don't think she would be a very playable character. She has to be in the right places at the right time and manipulate people. She's not like Guybrush, who is pleasant/pathetic and plays the thinks-he's-sneaky kleptomaniac game, then uses items in bizzarre ways to do things. I reckon other people know he's doing these things and avoid him because he's weird. Elaine is more of your typical woman.
LeChuck, on the other hand, would be great fun to play, as I'd imagine everyone would have fun thinking in a scheming, diabolical way, fuelled by misendevours, hatred and simply having the pirate-captain-to-rule-them-all psyche. The more I think about it, the Monkey Island: Legends of LeChuck, turning the course of the episodic model back to short stories that don't have to be connected is a great idea for one game. I'm certainly getting bored of the current Guybrush-LeChuck-Elaine dynamic now.
As for playing other characters not being MI enough, it's still within the MI Universe and it needs expanding.
Life is like an adventure game: if you don't click, you don't get it