ALTERNATE PARK-WIDE CONCEPT
Since we are in wild brainstorming mode, I thought I'd toss out another crazed idea, one which actually goes against many of the directions we might now be headed, but still preserves the castle, Fantasyland, Fantasia Gardens and Villains Land, at the very least.
Proposal: Since Fantasia Castle is the central park icon...what if the entire park related more directly back to the castle, and it all took place in that castle's "universe?" (Like how Paris' Frontierland has an overarching narrative to connect Big Thunder, Phantom Manor and the entire town.)
So the whole park has a sort of medieval/Renaissance high fantasy setting, with updated versions of all the classic "castle park" lands to reflect this. So Fantasyland becomes Fantasialand. Tomorrowland becomes, say, Cloudcuckooland, based on futuristic concepts of Da Vinci's time (in the manner of Fortress Explorations), all magnets and clockwork and alchemy. Frontierland becomes Explorationland, examining the New World from an Age of Exploration standpoint. Critter Country becomes Beastlie Countrie - imagine a Splash Mountain with unicorns, griffins and basilisks! The idea is still percolating - I have no permanent idea yet for Adventureland, and I could already write pages on the new Utopian version of Main Street and Yensid's castle backstory - so I'll just toss it out there right now.
Granted we can't use futuristic sci-fi, the World's Fair, or most IPs, but it's brainstorming! Going bold, as Tom proposed.