Some villain (for whatever reason) deciding to hasten the X-Blade's forging by going back in time to when the Destiny Islands fell into Darkness. And prevent Kairi's heart from migrating to Sora. Sora, Riku, Kairi, Mickey, Donald, and Goofy have to stop the villain, as they likely prefer the past to stay where it is.
The heroes arriving moments before KH1 Riku opened to door to darkness. Just in time to see the villain sneaking around while staying out of KH1 Ansem SoD and Riku's sight. As well as the former heading toward where DDD Sora and DDD Riku are.
Most likely scenario. Seeing the islands fall to darkness from Riku and Kairi's perspective.
Either Sora, Riku, or Kairi spotting KH1 Sora and KH1 Riku conversing. Before getting engulfed in Darkness, and KH1 Sora getting his hands on the Kingdom Key. And the hero witnessing this takes it as a sign that there's not much time, and calls their friends to warn them of it.
A revelation that the trip is actually a Stable Time Loop (look it up on TV Tropes), like ReMind. The door that appeared at the Secret Place entrance: That was meant to keep KH1 Sora from entering. Which is unlocked by the hero witnessing KH1 Sora receiving the keyblade. Kairi looking sick when Sora finds her: The villain knocked her out. Before they send her to Hallow Bastion, the heroes arrive and push them toward the room behind the door shaped keyhole. Donald then casts a lower class cure on KH1 Kairi before closing the door shaped keyhole (but not completely). The gust of wind that came when the door shaped keyhole burst open: The villain tries using some sort of attack on the heroes in order to finish them off. Mickey then casts Stopza, just as KH1 Sora was about to catch KH1 Kairi. KH1 Kairi vanishing just before KH1 Sora can catch her: The heroes grab her while Stopza is in effect, and Sora did the same thing he did for Namine in ReMind. Which explains how KH1 Kairi's heart migrated to KH1 Sora. The heroes then used an open corridor of darkness to send KH1 Kairi to wherever KH1 Maleficent will find her. This revelation will give at least Sora a minor headache.
Continuing from this. https://www.kh13.com/forums/topic/124514-idea-for-a-time-travel-based-game/
What do you think?