Very interesting idea. I thought you would focus on the factory part for a second!

The great Astral Serpent is very much alive, dormant for the next few billion millennia. Problem is, hilariously enough, it has a toothache because some parasitic worms have been burrowing around in its tooth to make their nest. The boss itself is the under-developed egg of the parasite matriarch, which has four tanky guardian worms defending it. Getting rid of the worms grants the party the Serpent's Blessing, which allows them to safely travel to the Astral Plane through rifts.

Here's my setting: A newly constructed tower, a futuristic technological wonder the likes of which have never been seen in the kingdom before.

this party is gonna think everything will be infested with worms. EVERYTHING. NO ESCAPE.

this tower has tons of cool stuff inside of it, including things capable of destroying entire kingdoms. (AKA, Atom bombs) the party was hired by a lord to investigate it. this party has had some not so good experiences with this lord, including a betrayal or two. the tower mysteriously appeared after creatures seemingly made of light 'constructed' it. its front door has a warning sign that reads, "do not use any item inside of this building for the whims of a single or a few individuals." now, the lord that hired the party is gonna double cross them, and the party knows this. however, if they use the bombs and stuff inside the tower against the baron, they will become public enemy number 1, and will be mercilessly hunted down by the entire kingdom's guard for the murder of one of their rulers. turns out, the enemy was greed itself.

my setting is: A shadow crossing overrun by sorrowsworn.

A shadarḱai gloomweavewr with a bunch of skulks and wereraven guards, that attack when you accidentaly fall into shadowfell.

Next setting. a forest dieing from a plague

Malu'rokk, a giant spider with a headband of intellect. It can write in Druidic, and spreads promises of cures for the sick and blessings for the cursed. In truth, it possesses a single guidebook to minor curatives, and believes this to be a master tome of all medical knowledge. Its most common prescription is bloodletting. It has no patience for any affront to its genius.

Next up! A dollhouse that's been magically enlarged to function as a real house.

a halfling lich tinkerer that has a new spell that turns you into a pig.

next: going basic with this one A volcano with bodies scattered all around

A golem compiled of various charred corpses mashed together into a three-armed humanoid form. As it takes more damage, it heats up and deals increasing amounts of fire damage to anything it attacks.

Next setting: An enormous hedge maze in the center of a dark forest.

That sounds grisly. I hope it doesn't explode, but I bet it does.

The minotree is a relentless pursuant that appears whenever anyone in the party turns around in the maze. It moves slowly like most plants, but the maze slows down its prey with difficult plant terrain. If killed, the minotree explodes into seeds that will grow into mini-minotrees within 5 minutes. 1d6 of these seeds land on the party, and 2d6 land nearby.

Next up: the top of a speeding train.

A clever twist on an old classic: I love it!

A flock of wyverns is attempting to tear open the train cars and eat the passengers and livestock within. Not as much a boss battle as an endurance test, as you don't need to kill the wyverns: You just need to keep them from eating people until you make it to a tunnel through a mountain where the wyverns can't fit.

Next setting: A sinkhole in the middle of a desert, where sand endlessly pours from above into it.

But can we give one of the wyverns a gun? :P

The mechanized security system of a crashed space ship. Although the crew are all dead, the system -- and its autonomous drone squad -- still fights off invaders using its laser weapons. These weapons crystallize the falling sand into pillars of glass, and get so hot during operation that the drones have a chance of bursting into flames.

Next up: inside an oversized froghemoth that's been temporarily paralyzed by magic.

The drones blowing up sounds hilarious!

The froghemoth is paralyzed because it's being researched in a magical institute, and was paralyzed so they could check on its health safely. In some mixup, the party swapped places with a few of the institute workers, and now they have to perform a routine check-up on this massive beast while keeping it sedated. Just make sure no one's in the mouth when the anesthetic wears off....

Next setting: A capsized galley on the bottom of the ocean.

The froghemoth might get a nice smack, does it like dwarf?

Dread pirate placeholder_name (I am too lazy to come up with one) had his treasure on that ship. The pile of magical gold pieces will move, attack, curse the players, and even reanimate the ship’s crew to fight the players. It can split into multiple smaller piles if it needs to.

Next setting: a cave inside a bottomless pit.

I had players fight some gold once. Well, technically it was air elementals whirling a pile of gold coins around. Still, definitely spooked 'em.

A solitary modron has been stationed in this cave for a hundred years, raising silkworms and spinning their silk into a long rope. Using a complex machine, it lowers the rope down into the pit steadily in order to measure its depths. So far, it has not reached the bottom. The modron is specially equipped with the magic needed to feed the worms, as well as to defend them from threats. It goes absolutely nuclear on any creature who threatens the project, casting Finger of Death as a legendary action if a creature ends its turn close to the rope or the worms.

Next up: The gravity-reversed underside of a spelljamming ship whose crew doesn't know you're there.

Cool idea!

The ship is being piloted by a chaotic evil captain bent on the destruction of the kingdom the party is from, they must find an air duct that cycles used oxygen hidden underneath a loose plate on the underside of the ship, crawl through the vents, find the captain and kill him, all while avoiding being detected by the crew or captain. 

Lizardfolk temple dedicated to Semuayna, lizardfolk goddess of survival.

The Lizard Queen Uelibloom, and her tamed T-Rex Orchid, who fight the party in tandem. Uelibloom has a few levels in Cleric, and she uses her powers to heal her pet. If Orchid dies, Uelibloom surrenders. However, if Uelibloom dies first, Orchid enters a Frenzy (like the Berserker subclass) and fights until killed. The fight has different difficulties depending on which target they prioritize.

Next setting: The shore of an acidic lake.

a flock of dire corbies enslave by an illithid colony make their home here to defend a new monster. the acid elder brain. several of the corbies are actually doppelgangers, and the bats flying around are druids also enslaved.

next setting: a railroad track that is mysteriously abandoned.

Nice, sounds like absolute chaos lol.

The ghosts of indentured railway workers haunt this place after being drained of their life force by their boss, a vampire. The sound of their hammers banging away, and their ethereal singing, can still be heard, and requires a saving throw against picking up a hammer and going to work on the railroad, as per the geas spell.

Every night, the spectral train returns -- half real, half ethereal, it can be looked through like glass, and it's difficult to see from a distance. The vampire boss is driving, and slaying him ends the geas. When the train passes by, all creatures save against exhaustion as the vampire attempts to drain their life force.

Next up: A lake, frozen over in the middle of summer.

That!s a good boss. The story is the best part.

A monstrous ice golem created by an ancient white dragon before breaking free from its master. Its attacks not only deal damage, but also create ice that eventually restrains them, but can be removed by enough fire damage.

Next: the intersection of a desert, a forest, and a tundra.

Sounds interesting!

The dragon Calyros has made its home here. Given how his mother was a blue dragon, and his father was a white dragon, he decided to terraform the land around his lair to fit both aspects of his nature (the forest being the result of two very different climates coming together). His breath weapon scatters electrically-charged ice that occasionally create arcs of lightning when something comes near.

Next setting: An old, abandoned mead hall.

Electrical ice! Love it!

The hall is treated like a haunted house by local goblins, who dare one another to go into it from time to time. They invariably get eaten by the massive otyugh that resides there. Centuries ago, the dwarves who built the place brewed a magical mead that can restore one's health, and the otyugh consumed it as well as the dwarves. It has the regenerative powers of a hydra, and has grown to ridiculous size.

The secret to brewing the magical mead can be pieced together if the monster is defeated.

Next up: a 50ft-diameter steel bowl, perfectly balanced (by magic?) on the peak of a mountain.

Sounds like fun!

A gang of three galeb-durs are using the bowl to roll around, doing tricks and showing off. The village at the bottom of the mountain wants the party to get rid of these hoodlums because they fear that the elementals might cause the bowl to fall and possibly destroy the village.

The fight itself involves the galeb-durs taking turns rolling towards the party or doing tricks. They don't see the party as a threat, and just want to show off their moves.

Next setting: A tiny hole in a inn's basement.