In order to give the planes more of a vibe - create sorts of bonuses/penalties/influences for when players are in them - and apply certain affects. I will drop a few ideas for places where players actually spend a lot of time at but the Book I mentioned, has a lot of cool ideas that could enhance planar travel without being torture.
SHADOW PLANE:
* Shadow Conjuration / Shadow Evocation / Shadow Descriptor spells should be considered enhanced. I'd probably make the Shadow Conjuration / Shadow Evocation line both empowered/maximized/extended since they are meme spells. Other similar spells should probably just be extended or empowered.
Enhanced Magic
Spells with the shadow descriptor are enhanced on the Plane of Shadow. Such spells are cast as though they were prepared with the Maximize Spell feat, without requiring higher spell slots.Furthermore, specific spells gain increased power on the Plane of Shadow:
Shadow conjuration and shadow evocation spells are 30% as powerful as the conjurations and evocations they mimic (up from 20%).
Greater shadow conjuration and greater shadow evocation are 60% as powerful (not 40%).
A shade spell conjures at 90% of the power of the original (up from 60%).
To calculate the effects of such spells, utilize the Maximize Spell feat to obtain maximum hit points or maximum damage, then apply the percentages above.
* Make light/fired based / producing spells fail on a spellcraft check.
Impeded Magic: Spells that use or generate light or
fire may fizzle when cast on the Plane of Shadow. A
spellcaster attempting a spell with the light or fire
descriptor must succeed at a Spellcraft check (DC 15 +
the level of the spell). Spells that produce light are less
effective in general, because all light sources have their
ranges halved on the Plane of Shadow.
Despite the dark nature of the Plane of Shadow,
spells that produce, use, or manipulate darkness itself
are unaffected by the plane.
* Consider maybe portions of the domain to have a negative energy trait in minor area damage effects, tiny bonuses to undead monsters / players.
No Elemental or Energy Traits: Some small regions
on the Plane of Shadow (called Darklands; see below)
have the minor negative-dominant trait, however.On the Plane of Shadow, patches of darkness exist that are
even darker than the shadows themselves. Negative
energy infuses these places. Whether the negative energy
is bleeding over from the Negative Energy Plane or it’s
simply a byproduct of the high concentration of undead
shadows and other life-draining undead is unknown.
Darklands regions have the minor negative-dominant
trait. Living creatures that enter the darklands take 1d6
points of damage per round from negative energy and
crumble into ash if they reach 0 hit points or less. Spells
and magic items that protect against negative energy
function normally within the darklands.
Travelers to the Plane of Shadow can identify darklands easily. They are more desolate and bleak than the
surrounding areas (which are themselves pretty bleak on
this plane). Plants are dead and desiccated from the longterm effects of the negative energy.
Natural vortices from other planes do not open into
darklands regions, and spells or spell-like abilities opening a portal from the Material Plane warn the traveler
(usually with a cold shiver down the spine) that a darklands region lies ahead. Darklands often correspond to
haunted battlefields, unconsecrated graveyards, and lairs
of powerful necromancers on the Material Plane, as well
as any location dominated by undead.
* Sanctuary/Gsanc don't work - anything that uses the ethereal plane.
The Plane of Shadow does not connect to the Ethereal
Plane. Spells and spell-like abilities that use or access the
Ethereal Plane do not function in the Plane of Shadow.
The Plane of Shadow is coexistent with the Astral Plane,
so various spells and portals make it possible to move
between the two planes.
* Maybe some sort of "if you don't have darkvision, ultravision, light source everything has concealment" as a cool little impediment. Also make light sources half their radius.
Shadow Combat: Given the limitations of vision and
magic noted above, combat on the Plane of Shadow is
otherwise normal. Shadow combatants fight in a universe that is eternally in night.Darkvision is unaffected
by the plane, but every torch, lantern, and light-emitting
spell (such as continual flame and daylight) has its radius of
illumination halved.
* Shadow Quakes.
The Plane of Shadow is a morphic landscape, but in general it moves slowly. Over the course of a week the landscape may alter sufficiently to be unrecognizable,
although someone continually observing the plane
wouldn’t see it actually moving. But the Plane of Shadow
has its own version of earthquakes that can prove deadly
to the shadow traveler.
Shadow quakes tend to be dramatic but localized,
having the same effect as an earthquake spell within a
100-foot radius. Flying and incorporeal creatures are
unaffected.
* Shadow Dancers should be able to teleport to internal Shadow Plane Portals while in the shadow plane as if with -teleport. There's a variant rule of enhancing their powers there and it could be a neat cookie for an invested SD.
NINE HELLS OF BAATOR:
* Provide discomfort and disadvantage to people in the server areas of hell. Make persuasion/intimidate/appraise checks with NPCs not as well off. More of a vibe thing (and personal), ie other players shouldn't notice you're going through the struggle except via RP.
Mildly Law-Aligned and Mildly Evil-Aligned:
Chaotic characters in the Nine Hells suffer a –2
penalty on all Charisma-based checks, as do good
characters. Chaotic good characters suffer a –4
penalty on all Charisma-based checks.
* Avernus actually already has its fireball thing. Artillery streams through the skies there and randomly lands on people.
* Dis apparently has spicy iron. Maybe put some area traits for being barefoot or touching metal objects.
Every iron wall glows with heat, and casual contact
with the walls deals 1d6 points of fire damage.
Even the iron cobblestones glow with heat. Without
iron-shod boots, visitors soon writhe and burn in the
streets.
* Minauros has acid rain and minerals so its good.
THE ABYSS:
* There isn't any non-violent content in the Abyss, closest is the citadel. But could provide similar discomfort to players in those places in hell. Could also give Paush's lair its own specific bonuses/penalties as its entirely Arelith lore.
Mildly Chaos-Aligned and Mildly Evil-Aligned:
Lawful characters in the Abyss suffer a –2 penalty on
all Charisma-based checks, and good characters suffer
the same penalty. Lawful good characters suffer a –4
penalty on all Charisma-based checks.