What's behind the door in Destiny Islands?

[Onion Knight]

I'm sure you all remember the mysterious door from the Secret Place. But the question is - what's behind it? My theory is that it's Kingdom Key, Sora's original Keyblade. In the original Kingdom Hearts, if the Destiny Islands got attacked by the Heartless, that means the world's barrier shattered. If the world's barrier shattered, that means its Keyhole was unlocked. And if its Keyhole was unlocked, that means someone had a Keyblade. And it's probably Riku. My guess is that the hooded figure (Xehanort's heartless) took advantage over Riku's desire to leave the islands to convince him that darkness is nothing to fear, so he could eventually turn him into a Keyblade-wielding vessel. And for that, Riku needed a Keyblade. So how did he get it? By going into the door. That would explain why Sora got the Kingdom Key in the first place, since it jumped from Riku to Sora to avoid getting into the wrong hands.

There is further evidence to support this theory. In Kingdom Hearts 0.2 Birth by Sleep ~A Fragmentary Passage~, King Mickey found his Kingdom Key D behind the same door in the Realm of Darkness counterpart of Destiny Islands. Kingdom Key D is even described as a dark counterpart of Sora's Keyblade. And the Keyblades that were required to close the door between the Realms of Light and Darkness were the exact opposites of each other, too. So it's only safe to assume that Sora's Kingdom Key comes from the same place in the Realm of Light as Kingdom Key D from the Realm of Darkness.

 

[KeybladeKnightQ]

I’m pretty sure what’s behind it is the heart of Destiny Islands. We saw in a flashback that the door was the worlds keyhole.

 

[Onion Knight]

I’m pretty sure what’s behind it is the heart of Destiny Islands. We saw in a flashback that the door was the worlds keyhole.

Then how do you explain Kingdom Key D in the Realm of Darkness? There has to be a reason why it's there.