THE CORE COSMOLOGY:
01) The Material Planes are the foundation, core and source of all of creation.
02) There are three planar layers: Material- (core-), Inner-, and Outer-Planes.
03) The transitive planes emerge from the diffraction of the planar layers.
04) The Feywild is a Cluster-plane typically connecting around ten Materials.
05) The shadowfell isn't undead related but the underdark of the Feywild.
06) The Feywild can only be reached from the Material never the Ethereal.
07) The Ethereal isn't an overlap on another plane but a space between spaces.
08) The Inner- and all material-planes are reachable through the Ethereal.
09) The Ethereal isn't a traditional euclidian space-time but something else.
10) The Astral, the Outerplanes and the Nether aren't spaces but immateriums.
11) The immateriums can only be reached and travelled by projection magic.
12) The Nether is the lower dark part of the Astral defiled by the corrupters.
13) The Great Dark Outside, aka: the far realm.
ETHEREALNESS (spell; ph, p. 238):
Casting Time: 1 Action
Range: Self (Touch)
Components: V, S
When you cast the etherealness spell you turn invisible and can fly as per the fly spell. When you are one km removed from the earths surface you can shift into the ethereal-plane leaving the material-plane behind. Because this setting dosn't have an border ethereal that overlaps the material world you can't walk the world in "ethereal ghost form". For using the spell on the material plane the duration is "concentration, for up to 1 hour". Unless you "attack or cast a spell" from then on the duration is "concentration, for up to 1 min". There is no duration once you have entered the ethereal plane itself.
PLANESHIFT (spell; ph, p. 266):
Casting Time: Varies
Range: Self (Touch)
Components: See below
You must "walk in a certain pattern in a certain environment" in order to shift plane; You can only planeshift at a "softspot"; Your DM will decide where the "softspots" in your setting are located; The planeshifter spell wont let you travel the immateriums, you'll need the astral projection spells to do that.
* one dosn't gain any experience for adventures outlived in the astral, the outerplanes or the nether.