Warning: Kingdom Hearts II spoilers
It would need to tell an extremely condensed version of the story. It could start with a summary of the events of the first game, and some of the earlier events of KH II, on the water screens. Then, the stage lights up to reveal Roxas, having a usual day with the others. Then, everything around him freezes in place and Axel appears, asking Roxas to come with him if he wants to know the truth. Stage fades to black and the water screens come up again, showing the rest of the Organization pleased with the retreival, then switches to Sora, Donald, and Goofy arriving in Port Royal on the Gummi Ship for the DL version, and Land of Dragons for the WDW version. In the DL version, a cannon blast from the Interceptor would startle the audience, and we would see Sora's group along with Jack Sparrow attempting to fight off Barbossa's cursed pirates. In the WDW version a firework blast would reveal a battle between the chinese army and Shan-Yu's Heartless (just neoshadows instead of those little bird things, though). After Shan-yu is defeated, Mulan is injured and revealed to be a woman, leading to a sweet scene with Shang. In both versions, Sora would seal the keyhole and depart. This would lead to another projection, in which Sora recieves a transmission while in the gummi ship from Cid, asking them to return to Hallow Bastion quickly. They speed off, and small, lighted boats appear. Each one has two characters on it, and, like the current ones, displays a romantic scene. Each boat would represent a romance from the game, including Jack and Sally. After they've all passed, a scene similar to the "gathering of evil forces" in the current version would occur, with Oganization XIII instead of the Disney villians. Axel is the last member shown, and he looks somewhat concerned before the stage lights up again, revealing an immense battle with Sora, Donald, Goofy, King Mickey, Riku and Kairi(they didn't show up at this point in the game, however), Cloud, and Leon fighting the nobodies (they fought the heartless in the game, though). To their surprise, Axel appears and helps them, sacrificing himself to open a portal to the World that Never Was. Mickey and the others tell Sora and Riku to go on ahead, and Kairi assures them that they'll meet again soon. They go through the portal, and Sora finds himself locked in battle with Roxas (displayed on the water screen, and would happen exactly as in the game). Now, unlike in the game, Xemnas merges with the other remaining members of the organization and opens the doorway to Kingdom Hearts, which would take the place of the dragon. Sora and Riku agree to stand by one another no matter what happens, and go through the door. Now switching to the water screens, Riku merges with Sora, and Sora turns into his final form (once again, different from the game), then switches to the stage again as he battles and defeats Xemnas. Back to the screens, now he and Riku appear to be lost in Kingdom Hearts forever, but Sora says it won't be so bad as long as they have eachother. Suddenly, they see a passageway open in front of them, and walk towards it. The Steamboat Willie river boat (or the mark Twain at DL) rounds the corner, lit up with the powers of light and containing all the characters that made even small appearances in the games. King Mickey is seen at the helm as Steamboat Willie, and Sora, Donald, and Goofy also in their Timeless River forms (this Sora would be a "Head Character") as The instrumental of "Simple and Clean" plays. As soon as they're out of sight, Sora, Riku, Kairi, and King Mickey appear on the stage, conducting the powers of light, ending in a huge pyrotechnic display. The spotlight shines on Sora, who says "I know now more than ever... Kingdom Hearts is Light!" and plunges his Keyblade intothe ground, vanishing in a flash of light.
As much as I like this idea, there's probably no more than a 5% chance of this happening, in the American parks anyway. I could see something like this at Tokyo Disneysea.
Please be nice people, that took me forever to write.