Long-time fantasy gamers get the idea of “planes”. Each plane is its own reality, and it connects in some fashion with other planes, and they’re mostly just where high level characters go to kill things and steal their stuff. Simple enough.
How about we swap out planes for strings.
[Disclaimer – I don’t know anything about actual string theory. I’m just stealing a term because it sounds cool and kind of fits this conception of planes and dimensions in fantasy games]
No, the elemental plane of fire won’t look like a tightrope – it will still be an endless reality of fire and efreet. But it will functionally be a string, as will all the other planes. Dozens of planes, as strings, crisscrossing the multiverse and, at certain points, touching.
At a planar nexus, the reality of two planes (or more) are combined, forming a little pocket dimension (perhaps a planet) that combines the aspects of the planes that are connected. So you might have a plane that combines Elemental Fire with Chaotic Good, or a plane that combines Shadow with Water. Maybe the Astral Plane is where all these strings are located – and maybe when traversing that plane you can accidentally hit a string and find yourself in another plane. The Ethereal is just the blurry edges of the vibrating Material Plane, and can be a passage to these “nexus points” that interact with the Material Plane.
Maybe the Material Plane is just a nexus point between all the different planes of reality?