Featureless expanses of earth, air, fire, and water are just not that interesting, even liberally sprinkled with elementals. That’s a core problem with most of the D&D planes of existence – they’re more like allegories than locations. Unless you’re playing Pilgrim’s Progress: The RPG, allegories probably don’t feature heavily in your weekly game.
In my opinion, the best planes are the ones you can wander into unawares: the faerie kingdom, the land of the dead, dreamland: and the inhabitants will seem strange and frightening, and the rules will not be the rules you know, but they will be close enough that you won’t have to wear a space suit.
With that in mind, here’s my attempt to fix the elemental planes: earth, air, water, and fire.
Earth: The plane of earth is no fun because there’s nothing to do except get encased in solid rock. What if, instead, it’s a vast megadungeon, aware and malevolent like the dungeons of OD&D? Like all the best planes, it has its own rules: that everyone but you can see in the dark, and that doors that stick for you open easily for monsters. In fact, many dungeon crawl campaigns might as well be set in the Plane of Earth, except that the players occasionally “go to town” to rest and sell their loot. This fabled “town” might be one of the strange bubbles in the Plane of Earth, little places where people live in the illusion that there is a whole aboveground world around them.
How can you wander into the Plane of Earth accidentally? A lot of dungeons are filled with pits, and some of the pits are bottomless. Bottomless pits drop you into the Plane of Earth. You could keep falling in such a pit for minutes or days: you stop when you successfully grab at a door or ledge along the side of the pit. (Long drops are common in the Plane of Earth, and the rules of the plane are such that an otherwise deadly fall always leaves you with 1 HP). Falling for miles is easy: finding your way back up to the real world will be a Herculean task. Depending on how far you fell, you might have to adventure your way up past dozens or thousands of dungeon levels to find the portal you fell through. How’s that for claustrophobia?
Air: Just as the Plane of Earth is below us, The Plane of Air shouldn’t be an infinite, featureless expanse: it’s in the sky. I assume that we’ve all looked down at the clouds out of the window of an airplane, and imagined striding across them like giants. But even in the world of D&D, clouds aren’t usually solid.
When you travel to the Plane of Air, the natural world becomes insubstantial, and you start to gently ascend as if on an air current. Cloudstuff is the only thing that you can touch. The clouds are constantly changing, their castles and villages appearing and disappearing, and the creatures of the clouds come and go too: you might see a cloud deer emerge from the billowing ground, run from a cloud wolf, and then dissolve, and leave not a rack behind.
Furthermore, when you’re on the clouds, you can interact with the storm giants. In normal life, storm giants cannot physically attack or be attacked by the creatures of the natural world. (They can, however, throw lightning bolts at the creatures of the prime material plane.)
Fire: For this one, I’ll use an idea I mentioned before: of a campaign world where fire was sentient, and had lineage. A fire lit by another fire would share many of its characteristics, as a child does of its parent.
Fires in this world could level up: a level-one fire would be one that was just lit for the first time, and would have no special powers. A level-twenty fire might have an intelligence, wisdom, and charisma of 20, and a bunch of special powers: telepathy, the ability to burn without consuming fuel, and the ability to burn with blue cold.
Rather than an endless plain of flames and lava, the Plane of Fire would be a world that was dark in many places: with no sun or moon, it would only lit by bonfires, the great Eternal Fires that rule kingdoms, and the torches borne by mortal slaves.
Water: I’ve racked my brain and I can’t think of a way to make a Plane of Water that’s significantly cooler than a garden-variety ocean. Sure, it could be infinite, but infinity is overrated. Just making something big doesn’t necessarily make it more interesting. So do you have any ideas?