As a GM, I love alternate planes and have used them for some of my most successful stories. To me, they represent an opportunity to throw out the normal rules, to define a custom reality, and to introduce story elements that are completely alien to the characters. I do this in two ways:
I take elements from another plane, and I have them intrude onto the material plane in ways that the players can see and interact with. This could be inhabitants of other worlds wandering into the character's realm with goals, searching or exploring, or even just lost. It could be natural elements that don't exist making their way through, such as a massive, but isolated rainstorm that occupies a 100' radius around a strange key. Or it could be something that is supposed to be bound to a plane that escapes into the character's path.
Alternately, I send the characters to another plane and have them deal with whatever they find there. This can involve figuring out the rules of a plane to be successful, solving the entry/exit criteria of the plane to move through it, dealing with inhabitants who are native to the plane and its rules, or any of a myriad of other ideas available through the magic of "It's whatever you want it to be."
I am especially fond of demi-planes; they are an amazing resource for storytelling. They can be custom designed for whatever rules and properties I need for the story, with bespoke entry and exit criteria. They require no more preparation than a normal adventure situation but can be as varied, surreal, well-defined, or loose as I want for the encounter--the rules are only what's defined by the plane.
For this reason, I was really excited to see Monte Cook Game's new Kickstarter, Planebreaker, which is being created with versions for both 5e and Cypher System. I think that they are uniquely situated to deliver something amazing in this space; the writers have already created a lot of planar content at TSR and Wizards of the Coast as part of the evolution of Dungeons and Dragons, through some Planescape books, Manual of the Planes, as so on. They also have a history of doing the work at MCG as well, through The Strange RPG, Worlds Numberless and Strange, and Into the Outside.
The concept of Planebreaker is that there is a moon-like object called the Planebreaker which crashes through planar boundaries in an attempt to escape something cataclysmic from before time. In crashing through, it creates scars, and brings through artifacts, incursions, and even pieces of those planes, along with objects that allow the holder to follow a path back through the scars or ahead to the Planebreaker.
This premise, as implemented as a Kickstarter, will deliver the Path (a GM reference), a Players Guide, and a Bestiary, along with a myriad of additional material like a GM screen, special dice, resource decks, and something I think is really interesting: Miniatures delivered in STL format for 3d printers. And of course, there are always stretch goals that either expand existing content or introduce new content.
Bringing Planes to the Game
One of the design goals is to ensure that Planes are accessible even to starting characters as low as 1st level. While I'm sure that exploring the plane of Fire will still result in a very fast 1st level party TPK, there is an appeal to a campaign that starts with "you inherit a coin" and use it to work your way along a path through low-level planes that are similar yet wildly different than what the players know.
The game is introducing new class archetypes, feats, items, and other player-facing aspects to the game, even before the planes themselves get involved. In fact, from what I can tell, it would be possible to run a full Planebreaker campaign and never leave the prime, just dealing with things from other realms that showed up.
But of course, there ARE all the new planes--and given the team's prior experience with creating new worlds, realms, and interesting places, I'm sure there will be a ton of content. Personally, and I'm only speculating here, I'm expecting some version of real-time content generators similar to what they did with Jade Colossus for Numenera.
The preview text gives a hint at what they're going to provide, and it checks all the boxes I'd expect from an MCG product: Player-facing content, GM content to reveal or use as unstated background material, opportunities for events, and points of interest. One concept I liked: A creature who can execute one perfect attack, at the cost of their own life. This is alien in both concept and the underlying culture that would create such an ability. And yet it's relatable.
The Evolution to Planebreaker
The Planebreaker idea, while unique and exciting, is a natural extension of what the team has done in the past.
Numenera contained the concepts of alien detritus, shrouded histories, and mysteries that could be pursued but never fully uncovered.
The Strange introduced the idea of a super-threat, a greater universe, world-jumping to places with different rules, and how a foreign world would play out in player-relatable ways.
Darkest House created the idea of a place that has its own rules and bizarre situations which could interact with visitors, then send them back.. Also, the transition or interaction with the foreign place often resulted in real changes to the character, before they were returned back.
And of course, the team has 2e-5e development, multiple planar works, Forgotten Realms, and the unique settings for Cypher system that each of them has developed.
Bruce Cordell, in his WebDM interview, said that one of their goals was to take something that everybody loved and create something both unique and interesting. From the Kickstarter text, the sample material, and the interviews I've seen so far, it looks like they're on the path to doing that.
I'm excited about the Kickstarter for the content, but I'm also excited because of the way MCG tends to approach new material. They deliver quality products, which is a minimum, but the people involved often go further and deliver a new way of thinking about things.
The Jade Colossus was a great Ruins generator for Numenera, but it also gave me a way to think about exploration. Going back to the 3.x material, the Psionic Handbook fixed a long-broken concept in D&D, but it also gave me a whole new way of looking at alternative magics in my own world. Even the Epic Level Handbook, which never came into play for any game I was running, gave me a system-independent way of looking at higher level characters and continuing to challenge them--even when they were within the "normal" levels the game supports.
I'm hoping that this new work will not only give me great adventure hooks but will give me new ways of looking at planar topologies and the possible uses for planes, regardless of what game I'm running.