Reading over this debate about the "closed" nature of the Outer Planes, I'm reminded of the introduction in Zombie Sky Press's
Along the Twisting Way: The Faerie Ring Campaign Guide (affiliate link), which posits that the standard Great Wheel cosmology is incomplete. There's the Outer Planes, the Inner Planes, the Transitive Planes, the Material Plane(s), and also...the Preternatural Planes.
Those last ones are, essentially, the planes which don't fit anywhere else. Places such as the Plane of Dreams, Purgatory, the Faerie Realms, etc. They were always there, but didn't fit into "standard" groupings of the planes, and are where a lot of the more unusual creatures in the multiverse, such as fey, come from.