So, I believe we already have the 3.5 update from 2E for entering a sleeping person's dreamscape *Heroes of Horror has the 3.5 update for the 2E Ravenloft/Guide to the Ethereal rules for Dreamscapes*
However, there are other uber-interesting, exotic "demiplane" and "pocket plane" ideas.
--Mindscape: Dragonlance: Towers of High Sorcery presents a spell called "Travel the Paths", which allows one to enter a world of a character's past memories *along with complete rules to go along with it*. This is perfect for combining with the "Flashback Campaign" rules from Eberron: Forge of War.
--Artscape: Necromancer's "Eldritch Sorcery" book presents a spell (Merge with Art) which allows a character to hide in a painting. However, that gave me the idea of pocket demiplanes called artscapes (I'm not much of a novel reader sadly, so the only example I can think of off my head is an old Darkwing Duck episode involving the villainess Splatter Phoenix, who sealed Gozallin, Drake Mallard's *Darkwing's true identity's adopted daughter* in a black-and-white Matisse-style painting. Over the course of the episode, Darkwing and Launchpad end up *using Splatter Phoenix's paint magic as I recall* leaping into various paintings, including one involving a Jurassic landscape, where they are nearly devoured by dinosaurs and swallowed by a volcanic eruption. Naturally, there was also a Scooby-Doo style chase scene in Escher's Relativty)
--Bookscape: The D&D worlds are already filled with instances of sealing monsters into enchanted tomes. A bookscape works best for books of fiction and history.
A few questions pop to mind:
-In the case of Artscapes and Bookscapes, are they shared realities?
For example, if the PCs were to get sucked into "Alice in Wonderland" (an excuse to tie in the classic module EX1 Dungeonland), would they potentially meet other real-world people(?) who were sucked into other copies of the book? Or is this pocket realm unique to the unique copy of the book? If the realm is shared and the PCs escape, into which copy would the PCs emerge (e.g. they might step out into a private library or into the nursery of a young prince)
For Artscapes, it might be different as there might be multiple paintings of "Atlantis" (or wherever) that define different "edges" of this shared realm
-In the case of Bookscapes, can the PCs change the story? E.g. if they defeat the Red Queen, would the book re-write itself with a different ending or would it just "reboot" once the PCs exit?
I guess a similar question would be, if you changed the painting (e.g. painted a pile of gold) would this alter the Artscape and would actions taken in the Artscape (e.g. solidifying the melting watches in a Dali paint using magic) cause the painting to change (e.g. the surreal landscape is now populated with normal looking watches)?
-What is the nature of objects taken from these realms? Do they remain intact upon leaving or do they exist only within the pocket realm? (E.g. the PCs might be tempted to keep re-reading "Jabberwocky" until the entire team had vorpal swords) I guess you could get around this by saying that the items "native" to the art or book disappear when you leave but that there are some treasures left behind by earlier visitors from the real world (e.g. a magical shield from someone who was NOT able to defeat the Jabberwocky and died)
-Do these realities "reboot" and if so, can the populace learn. E.g. if the PCs go through "Alice in Wonderland" once andthen go back a second time (if that is allowed) would their allies still remember them as allies or would they have to re-introduce themselves? If they trick a monster into a trap once, could they just follow the same steps and trap the beast again or would it remember and learn?