*finger guns* noice

goblin king. I got nothin else really I read labyrinth and remembered the movie labyrinth so yes, goblin king

Dessert temple rigged with explosives  

Very perilous...

The boss, Mistah Stabbs, is a delirious goblin who extends the Goblin Boss's Redirect Attack feature to work on saving throws, such as the ones you make when an explosion happens. Stabbs is barely aware of his surroundings on account of [dehydration / too much cake] and believes the party to be visiting him for a tea party. His teapot, if he can find the blasted thing, requires a very large flame...

A 20ft tall wooden cage that's being carried by a roc over a mountain range.

Meme boss. This is a compliment.

The roc’s master, a legendary warlord looking for a battle worthy of his might. Fight to the death, the roc will lower the cage after the battle.

The back of a tarrasque

Quote from KingOfCharge >>

Meme boss. This is a compliment.

The roc’s master, a legendary warlord looking for a battle worthy of his might. Fight to the death, the roc will lower the cage after the battle.

The back of a tarrasque

brilliant. 

After an earthquake revealing the tarrasque's back about a hundred years back, a lake formed on it, and eventually became to an Aboleth known as Cthulu. the Aboleth attempts to charm and enslave the players at first, but if it fights, it does so to kill.

the cave of a tribal shaman

A tribal shaman

Divine Beast Vah Medoh (Zelda: Breath of the Wild)

I don't know that game well, but that's a big bird that's also a dungeon, right? Something like that must require some strange magic. Behold:

Gravity Master Akoh, a mage-knight. He can reverse gravity inside his chamber at will, ignoring this shift at his own discretion, and he prefers to walk underneath his enemies before letting them fall upon his three-pointed spear. This massive weapon terminates in a stable tripod, and he doesn't thrust or throw it, but braces it on the ground pointing up. It can skewer three unlucky characters who are within 5ft of one another if they fall. His other hand casts spells.

Next up: A carousel that's so large as to make the characters seem like children.

Amazing!

A Dybuk that instead of possessing corpses possesses dreams. it will have the night hag ability, and once a short or long rest it can summon something from the party's dreams to fight for them. the carousel is a figment of their imagination, and is really just vines of poison ivy.

Next up: an island similar to Isla Nublar. (Jurassic world island)

Oh hey, this thread lives! I like the idea of the party suffering some kind of poison until they realize the nature of the arena and do something about it.

The boss is an evasive sneak type of mage, with the power to call volcanic meteors from the island's active volcano. He stalks the party and calls these strikes down on their location whenever they're doing something that takes time, like a ritual or a rest. The attack takes maybe five minutes to land, and anyone looking up has the potential to see what's happening in time to get away. The boss runs to a safe distance and hides after starting an attack. In a fair fight he's not that tough.

Next up: An unbelievably long hallway where the wall torches magically ignite when someone is close, and magically go out when no one is close.

The ghost of an ancient tyrant, which must be defeated before you can leave.

An arcane ship built to travel between the planes, crafted by a powerful mage.

Simple, yet effective. Good idea.

Astrum Argus, a giant cosmic worm that can tunnel through reality. It mainly attacks by ramming the ship or shooting crystal spines from its sides, and sees the party as an easy meal.

Here's my setting: A massive tower in the middle of a wasteland, patrolled by ghosts and fire elementals.

I like that, sounds like a fun boss.

An ancient red ghost dragon.

A tower so high its top is in space.

Not a place I'd like to visit!

Oa'sion is an elven druid who took up residence here long after the disaster. He used nature magic to create small livable spaces from which he could carry out his research. He helps the party during the day by providing information, healing, and water. However, he's possessed by an evil ghost, which controls him at night, when he poisons the party's water, steals weapons and foci, and even attacks them in their sleep. He can travel between his oasis zones using Transport Via Plants, and does so if caught.

Next up: An echoing cave with a magic crystal in the center that echoes spells.

Quote from KingOfCharge >>

I like that, sounds like a fun boss.

An ancient red ghost dragon.

A tower so high its top is in space.

 Whoops, I was still writing my previous comment when you posted yours lol. Here, I'll just... Go again?

Space Knight Baluk has a horrifying power: On a recharge of 5-6, he creates a ball of arcane force that lasts until his next turn. The ball has the image of a random city or settlement the PCs recognize. If he takes damage while holding the ball, it rockets out of his hand and off the side of the tower, plummeting towards the location shown on its surface and becoming a meteor.

Next up: an echoing cave with a magic crystal in the center that echoes spells.

Cool ideas! Here's mine:

The Crystal Heart itself is the boss, and it attacks in three main ways: It can cause crystals to rain from the ceiling and spawn Crystal Watchers (basically Nothics with a new coat of paint), but the most deadly attack it has is its Arcane Laser. When the Crystal Heart is attacked with a spell, it stores the spell's energy within itself. Once it builds up enough energy, it unleashes a devastating laser attack that if it reduces a party member to 0 health, kills them and petrifies them into crystal statues.

Here's my setting: The ruins of an enormous aviary, where various tropical birds still linger.

Ok, now THAT’s a boss fight.

A huge bird bred purely for combat. One of them still terrorizes the area and nests here.

A normal-looking house.

It's like a dragon, but more humiliating! Lol.

Inside the house are countless oddities: a crystal tablet that displays images of distant realms, a standing sarcophagus filled with elemental ice and preserved components, and a strange homunculus the approximate size and shape of a small shield that mindlessly wanders the floors, seemingly unaware of the party. And the master of this domain, a reclusive mage with a deadly scepter. He can simply point this weapon at a character and, in a crash of arcane thunder, their heart is pierced and they die. Despite this power, the mage is frightened and confused, speaking in a tongue foreign to this world.

Next up: A party member's hometown, in the midst of the most intense rainstorm in recorded history.

Cool idea! You can really go loads of different ways with this encounter.

At the heart of the storm slumbers the Storm Wyvern Egg, which can only hatch in the eye of a powerful hurricane. The egg itself is creating the storm in order to create the hurricane needed to hatch. However, the egg cannot be harmed until the eight Zephyrs (invisible stalkers that can cast lightning bolt) guarding the egg are destroyed. At that point, the egg begins summoning more Zephyrs while being vulnerable to attack. Destroying the egg disperses the storm.

The problem isn't fighting the Storm Wyvern Egg, it's reaching it, as it sits levitating in the center of the storm. The way the party can get up there is by convincing the silver dragon that lives nearby to aid them, a difficult task, and flying up with the dragon to the heart of the storm.

Here's my setting: The stomach of an old, ill, dying Tarrasque.

That is quite imaginative.

The stomach of the Tarrasque is infested with hundreds of thousands of flesh eating, green, undead Kyuss worms. in the center of it is a star spawn larva mage breeding the worms. In 1 week time, the Tarrasque will die, and the surrounding villages will fall to an invasive plague of evil worms. while in the stomach, the Larva mage has the fling worm ability from the spawn of Kyuss statblock, and can use it as a Legendary action. if they manage to reduce the Larva mage to 0 hitpoints, it turns into a spawn of kyuss that lives for 2d8 more rounds with the soul purpose of infecting one of the party members to live on.

My setting is: the Lair of a deceased Nagpa.

That's a really unique and fun-sounding idea!

Turns out, the Nagpa who lived there happened to moonlight as a necromancer. His remains became part of his greatest creation, the Bone Lord, a massive heap of skeletons in the crude shape of a four-armed humanoid. It mainly attacks by summoning skeletons and spewing fire, ice, or shadow from the dragon skulls on three of its arms. Its fourth holds an enormous spear forged from dragon bone, which it will use to skewer party members from over 30 feet away.

Here's my setting: The broken down remains of a cathedral, repurposed to be a factory for various machine parts.

real interesting! players won't expect it to have minions, hehe.

the players are hired to help prevent some mysterious force that is killing off the employees of the factory. they find 4 odd statues scattered around the place. If they investigate, they awaken a very angry Eidolon. it also has the ability to use cleric specific abilities, like channel divinity (take that, necromancers) based on the domain of the god that created it.

my setting is: the hollowed out tooth of, "the astral serpent."