I remember in MI2, LeChuck was almost like Sauron. The ultimate evil. The one you most definately didn't want to invite to your birthday party, because he'd almost certainly eat all the cake by himself. You know. Like really really evil.
LeChuck should've had the same status as Sauron did for LOTR. The one to avoid. Instead, he's become a common thugh you have to save the caribbean from every now and then. TOMI brought that back in the last part - too bad he looked like a rabbit with a beard, so I almost couldn't stop laughing during some of the cutscenes.
In short, the next Monkey Island, if they bring LeChuck back, it should be like a LOTR (a final epic journey to rid the world of LeChuck for good), but with brilliant Monkey Island humour. And just to stay on topic, Largo LaGrande should have a big part in the next game.