This is probably a theory but after watching this card story. I remember back at WxS main story where Tsukasa first time visiting the WONDERLAND SEKAI and he said he found the sekai familliar as if he's gone there before and the fact that Miku told him "You're back again!" as if Tsukasa already visit WONDERLAND SEKAI before the main story starts. What if the reason why Tsukasa felt so familliar and why Miku said that its because TSUKASA ALREADY DID VISIT THE WONDERLAND SEKAI AT THE YOUNG AGE, but the sekai was still a fragment SEKAI and he managed to visit it thru his dreams! Remember! Visiting a fragment sekai means having a strong feelings and emotions, since Tsukasa managed to make a SEKAI alone and even managed have 2 VIRTUAL SINGERS there before the main story starts
THANK YOU SO MUCH!! Also I loved how they portraited Kaito in this story, because that's how I always thought him. As someone who has a calm voice to support me in the moments that I'm anxious.