I love the historical/geographic setting of the Indiana Jones movies (at least the first two), but I doubt a successful MMO could be based on a storyline with just one main character and a minor sidekick. There just aren't a wide range of characters like there were in Star Wars to identify and build a character base from. A server full of kids running around with fedoras and whips doesn't cut it.

Monkey Island could be fun. Anything pirate-based, if done well, could be fun. Getting the balance right, so every player has something to do and can be a hero of some kind (even a minor kind, when leveling) is the tricky part.