I run my game with a few personal desires in mind. I like a game rooted in the physical environment, hand to hand tactical combat, weird fantasy, evocative people and places, and an encouragement of lateral thinking. I want the game to mostly revolve around exploration and interacting with a strange and evocative world, where the players are never entirely sure what to expect. Combat is tactical and dangerous, but is meant to be fun and accessible. There should be fights where the players can auto-pilot through it by clicking on the enemies, but there should also be dangerous and strange fights that punish players for failing to pay attention. Generally, encounters will be balanced through 5e numbers for the players' level and the number of players in the game that day, and I adjust challenge on the fly depending on everyone's mood. Boss fights should be dangerous stand alone encounters that can't be cheesed, though they might be able to be bypassed and avoided through careful thinking and negotiating. The physical environment should always be a hazard, from traps to damaging hazards to the walls itself. Any places or travel or dungeons should feel physically present, and anything that allows players to bypass physical obstacles are sparing and powerful.
Generally, if a spell or power ends up interfering with this, I'll alter or disallow it. I ended up disallowing Etherealness, the fifth edition spell that allows free travel through physical obstacles, because it would make exploring and traversal of the physical environment too risk free for my taste. My players also have a couple custom items that they've figured out interesting ways to use, for instance, they have a magic ring that opens a door to a small demiplane. Originally it was too hot in there to feasibly use, but they fixed that, and now they've been using it to hide prisoners, carry each other, and spy on their enemies in complex ways. This is great, but I put a couple limits on it to prevent combat from getting too far out of the bounds of what I want, for instance, I could imagine them jumping in and out of their demiplane during every fight. This is neat but it's just not what I want from Dungeons and Dragons.
Okay, but WHAT IF, actually, I went all the way with this stuff? What would it look like?
For instance, Divinity 2: Original Sin's combat and gameplay is heavily based around teleportation. At first maybe one or two characters has the ability to teleport, but eventually it turns into a crux of the game. You can teleport enemies into lava, you can teleport lava onto enemies, and enemies can do that same to you, eventually you can swap terrain around, and combat can alter dizzyingly in an instant based on this single game mechanic. You have to use it offensively and intelligently or you'll be just smeared by the opposition, or unable to overcome certain challenging encounters. I wouldn't want my game to look like that, but, WHAT IF?
I'm going to take a few of the game mechanics from my game and extrapolate it out as far as I can go.
These are two metal tea-pots. The top opening on each is about as big as an okay sign formed by a thumb and index finger, and they are connected at all times by a magic gate. Anything that passes through one comes out the other.
HOW ITS BEEN USED:
Aiming spells and arrows from a distance. A beefy tank runs up and aims one telepot, while an archer or spell caster stands far back and casts or shoots through theirs.
Escape and reinforcements. One telepot is left back at camp and manned by their henchmen. In case of emergency, the party can escape back to camp with a variety of close-range teleports. Alternatively, their reinforcements can cast spells, healing, or cure status effects through the teleport. They can also keep a close eye on their camp in case it's attacked.
Testing planar boundaries. Since planar boundaries can only be bypassed through magic more powerful than the telepots, they can quickly check if their telepots are working, and if they're not, they can figure they've wandered into another dimension (this happens more often than you'd think).
Bypassing difficult obstacles and traps. The person most suited to sneak past a horde or enemies or difficult trap can scout ahead, and when they're safely through, the rest of the party can use short-range teleports to come through. (The drawback is that the telepot is left behind, so they usually only use this in combination with the telepot left at camp)
BUT WHAT IF:
Enemies had telepots? Single or duo enemies routinely sneak up and aim their telepots, allowing the rest of them to safely unload ranged attacks and spells? Combat would require players to identify who has a telepot. Enemies would have to be careful not to let their telepot fall into the players' hands, to prevent them from gaining access to their reinforcements or camps. Enemies would try to break or steal player telepots, to gain entrance to camp and prevent stealth attacks.
Players without short-range teleports would be at a distinct disadvantage. Either classes would all need short-range teleports, or classes without them would be at a disadvantage, or they would need a big buff to make up for their lack of mobility. Preventing enemies from teleporting through small openings would end up being a big advantage: counterspells and teleport locks and the like. Enemies would do the same to players. They would use grapples, disarms, called shots, and telekinesis to seize any telepots they see.
I could imagine a chain of telepots!! One enemy has a telepot that leads back to a party of enemies, the party has another telepot that leads back to their base. I imagine the boss would be very wary of a telepot falling into enemy hands. They would probably have outer bases to scout from, and central HQ would probably have some kind of teleport seal on it to prevent enemies sneaking a telepot into the base (I've already done this with the TEMPLE OF AAMEUL, a cult well versed in the dangers and advantages of dimensional travel). Powerful enemies might secret their HQ on other planes, for the same reasons, and smart players would probably try to do the same.
If a telepot falls into enemy hands, the enemy would sabotage or destroy the connecting one, to prevent them from hopping through.
Area of effect spells could travel through telepots. Fireballs, gases, curses, emanations, and so on would be in danger of traveling through the opposing telepot. Anyone manning the telepot on the other side would want to stand at a safe distance or have protective magic. Conversely, a person or group could make themselves immune or resistant to a particular area of effect, advance with a telepot, and the wizard could cast the area of effect on their teleport, causing the person holding the other end to be surrounded in a damaging aura.
It would be plausible to imagine encounters unspooling out of no where and retreating, the entire game turning into a series of dimensional guerilla attacks, eventually aimed at finding a path into a secret dimensional fortress.
THE HOT CAR DIMENSION
This is a three-piece jewelry set of two strange metal necklaces and a ring with a keyhole in it. The strangely curved metal pieces on the necklaces fit together to form a key, which fits into the keyhole. A scalding hot metal doorknob appears hanging in the air, when turned, it opens a door into a small, scalding hot extradimensional metal room. You can't spend more than ten minutes in the room before suffering some kind of damage, asphyxiation or hit point damage or passing out or something. Once it's closed from the outside, the door is locked from the inside.
HOW IT'S BEEN USED
Recently, they found a delicate glass ornament that lowers the ambient temperature around it by about 50 degrees F. They put it in the hot car dimension to make it a more palatable 70 degrees or so, "The Balmy Car Dimension," etc etc. But before that it was a little more tricky.
Throw ice type enemies into it. They met frost demons only harmed by fire and heat, so they just grabbed the ice demons, threw them inside, and shut the door. Throw regenerating enemies into it: they found a group of enemies that resurrect from a spawning pit when they're killed, and figured out that the planar boundary would block their soul's travel, so they threw those enemies into the hot car dimension right before killing them.
Now that it's balmy, they do teleporting tricks. If they need to teleport a mid-range distance, say across the dungeon, two of them get in the hot car dimension and the wizard Dimension Doors with the jewelry.
Storage for inconvenient hostages. In my game, vampires are paralyzed but not killed when staked, so they'll stake vampires and hide them in the hot car dimension. If they need to kill a vampire, they'll do it in the hot car dimension so as not to leave evidence in the dungeon. In a difficult fight with an enemy party, they petrified the thief, pushed him out a window into the hot car dimension, and closed the door, and used him as a powerful bargaining trick in later negotiations.
BUT WHAT IF?
In some ways, this might just look a lot like the telepot situations. Enemies would have an easy time teleporting reinforcements in. Single patrols of sneaky enemies could, again, quickly produce a squad of enemies. Players would probably end up paranoid of single enemies.
What if enemies kept hazards or dangerous monsters in their hot car dimensions? Maybe they fill it with poison gas or slime, and then open it as a kind of spell? What if enemies kidnapped players and put them in their own hot car dimensions? Players would have to have ready ways to get out of other dimensions, demiplane or plane shift spells.
Bedrooms and apartments in the hot car dimensions. They could go in there and read books. Enemies would have little apartments just for them, existing in the infinite null-space that extends in all secret directions.
See, my brain just doesn't work tactically this way, I can't care for long enough.
POTION OF IMMUNITY TO YELLOW
Does what it says on the tin. Here's a potion that confers full damage immunity to anything colored yellow, for 1-6 minutes.
HOW IT'S BEEN USED
So far, my players have not drunk this potion. But they bought a can of yellow paint. It was an ordeal. They have to go to some fucked up slum filled with fly people to find an alchemist who also tried to sell them a flailceratops. They seriously considered it. But they only have one potion and one can of paint, so they haven't used it yet.
BUT WHAT IF?
Okay, so what if color immunity was more common? Low level spells grant resistance and immunity to certain colors. At that point players would certainly be trying to manipulate color. Paint, dyes, clothes, illusions, and so on. There would probably be spells to manipulate color. Turn an enemy this color, swap colors, and so on. Enemies would have color resistance and immunity. Enemies would area of effect color change spells, spells to swap color, abilities to change their own color. Chameleon type enemies would be more of a problem. Items and spells that clean you of dyes and paints would be crucial. Solvents and paint-thinners would become essential adventuring equipment. Outer coats of different colors based on enemy defenses would be necessary. If you get dyed a certain color, just put on your vantablack trench coat and swap out your sheathed sword.
I might do more of these later.