The Universe

The universe that this game is set in is laid out below:

The Planes of Existence
All of the planes sit on the Great Wheel.

Spells
There are spells that will work between Planes. Everything that is listed in the PHB for planar travel is allowed in this realm.

Portals
There are also portals that are difficult to find and require a legendary ability to create.

To create a portal depends on where it is you want to go. In the world of the Great Wheel, all portals are circular and on sacred land.

Standing portals have similarities to the plane it accesses, guards

Within the Inner Planes
Then there are a few places where characters can simply walk. There are heavily guarded and difficult to find but with research and a strong adventuring party, you can make your way there.

Shadowfell has a link in the Underdark, while the Feywild has a link in the depths of Du Weldenvarden.

The elemental planes each have a sacred spot within the material plane which it is possible to cross but incredibly dangerous.

The River Styx
The River Styx traverses all of the lower planes. Once you make it here you can get from one lower plane to the next.

Inner Planes
This consists of the material plane, surrounded by the elemental planes. The farther to the out edges of the elements the less hospitable. When PCs visit the elemental planes it will be in the border planes near the material plane.

The Material Plane
This is where the bulk of the game takes place. It is the human realm and the plane that Graz'zt wants to absorb into the Abyss.

Feywild
To enter via portal it only works at dusk (the time it perpetually appears on this plane) on a specific day. On the continent let's put only one portal in Du Weldenvarden. Places of near-perfect beauty.

Time moves differently here. So roll on the time table to see how much time has been lost. NOTE: think about how that affects the master plan if it's too much place a friendly NPC to grant a wish to move time backward. Also, we'd need a reason the PCs would care that the time has shifted. If they are there too long then the world would have changed and the portal might not work. Is there a reason the Fey would want to maintain this portal?

Shadowfell
Plane of undead, darkness, and shadow. The portal crossing here is from the dream I had, the tomb underneath the island of TKTKTKTK.

Tomb guarded by a religious sect of Drow elves that worship Shevarash

We'll try to keep the Party out of Shadowfell, Strahd the first vampire rules the part of the plain they would enter veering into a new campaign BUT if they do then we'll look into Curse of Strahd campaign. Graz'zt would make a deal with Strahd to murder this nuisance party.

Elemental Chaos
At the farthest extents of the Elemental Planes, the pure elements dissolve and bleed together into an unending tumult of clashing energies and colliding substance called the Elemental Chaos. Elementals can be found here as well, but they usually don’t stay long, preferring the comfort of their native planes. Reports indicate the existence of weird hybrid elementals native to the Elemental Chaos, but such creatures are seldom seen on other planes.

Air, Earth, Fire, Water
In the event that the PCs travel here, I'll reference this DnD Beyond Article.

Portals:

Air -- Tallest of the Boer Mountains with a village of aarakocra.

Earth -- entrance in a grand cavern in the underdark, think Platform 9 and 3/4, entrance through a grand stalagmite like the base of the Spire.

Fire -- Molten caverns of the largest volcano in the northeastern part of the map. From Daevabad there is a portal that will bring you to this volcano chain.

Water -- Portal is on the back of a giant dragon turtle with a magical artifact calcified to his back. By touching it and saying the magic phrase in Primordial. Also, an especially powerful El Nino will open a portal, once a millennia give or take a century.