The Reverie Corridor
Daydreams, extra characters and such are linked to Reverie progression.
You get told fairly early on that you only can get the silver loot spheres by repeating the same level of a corridor when you already got the higher tier ones that give you new characters and such.
Daydreams are purely locked to Reverie progression, to unlock every daydream you have to complete the entire Reverie corridor and also beat the final secret boss that you can only challenge after the main story has concluded already.
You will also regularly be locked out from further Reverie progression until you advanced the main story in the real world. And the other way around.
Basically if you want to unlock everything you have to finish the main story and the complete corridor.