I know you're just kidding but in concept this couldn't work because Legoland is a gated theme park and we must not put any barriers between Marketplace and the West Side. That's how we got into this whole debacle to begin with!
We live about an hour away from the Legoland in Windsor, England.
It's certinaly a park more for youngsters with most of the rides suitable for those between 2 and 12.
I think it could be an excellent addition to the Orlando theme parks, however it would have to very very well done to convince people to give up a day of a Disney holiday.
I've been to Legoland California and it's definitely not worth the $50+ admission. However, if they think they can make a profit in Florida, being next to WDW can't hurt.