Addon: Multidimension

The first dimension is the cave dimension, being challenging but rewarding.
First, the way you get here is with 10 cobblestone and dropping in a lava bucket in one of the corners.
Once that is done, it will look like this: 

When you enter, you will be greeted by a lot of ores, but a ton of wither skeleton spawners. Spiders also spawn here naturally. 

It simply rewards you with ores for coming here. However, you will find a dangerous boss known as the ancient cavern spider. It has 700 health and becomes stronger and more defensive at half health. It behaves like a normal but buffed-up spider but will strike you with lightning and a poison spell. It also has poisonous bites. It deals over 20 damage and can kill you easily if unprepared. 
The next dimension is the sky or skylands dimension. It is simply a small area above the sky that can be reached with 10 blocks of quartz and throwing a torch in one of the corners.

Upon entering, you will find yourself somewhere where zombie piglins will naturally spawn, an oddly mysterious temple, and maybe even a few rich resources. 

The next is quite an unforgiving dimension: the temple dimension. It is reached with 10 sandstone and regular flint dropped into one of the corners.

When entered, you may be greeted by a few naturally spawning husks, towers with strays that house no loot, and a large temple that houses the Firic Mummy guarded to the brim with skeletons. 

Inside the pyramid is the firic mummy that waits for you. He is immovable, will keep blasting out shockwaves to send you flying a bit back, and will shoot intense rounds of fireballs at you. He has 600 health and will be granted a shield at half health. 
 
Last up is the toughest dimension: the ruins. It is reached with 10 stone bricks and throwing lapis into one of the corners. 

When entering, you will be greeted with an area of no ores and only what remains of a no-longer-existing world. This place only serves as evidence of what used to be a nice place to visit. Now it is barren and darker than it was. It simply has naturally spawning ruin golems that sometimes fight each other and may even accidentally knock you away. 

The soul fire arena leads to the hardest boss: The Skeletal Dragon. It can't fly anymore, but has 800 health and only becomes stronger and more resistant at half health. He deals 25-30 damage per hit. He will also strike you with lightning and summon large exploding bones above you. He's dangerously quick, so come prepared. 
Each boss has their own loot:
Ancient cavern spider: iron block, emerald block, carvern staff
Temple's Death Worn: diamond block, iron block, temple staff
Skeletal Dragon: netherite block, ruins staff
Sky Dragon: diamond block x2, emerald block x2, sky staff


V2 UPDATE:

New boss: Sky dragon. He is a stationary boss with 800 health and gaining of a defense boost at half health. He spews out lines of sky orbs. He summons shockwaves with devastating 40 damage and will spawn targeted chains of lightning on the player. He is found in the skylands.
 

Firic mummy now has a phase 2 that spews out poison and shockwaves. The temple will be destroyed and the "Temple's Death Worn". And before you ask, YES the death "worn" instead of death "worm" is intentional. Otherwise, he has 600 health and a defense boost at half. New items:
First, the iginition items for portals has been changed to these craftable items.
 (portal keys added for v3)(multidimenional rock added for v4.5)

BUT, there are also staffs you can get from the bosses as well. The death worm drops the temple staff that gives you regen and turns you invisible as a getaway device. The cavern spider will give you a staff that freezes nearby enemies and effects them with a poison spell. The skeletal dragon will drop the ruins staff which summons a bone projectile above enemies. The sky dragon will drop a sky staff that summons chain lightning on nearby enemies.

These items are NOT craftable. They may have infinite durability, but they have a large cooldown.

V3 UPDATE:
The new update features some simple ability changes to bosses and some brand new textures. Mostly bug fixes and better gameplay. 

V4 update:
This update brings a new ore, compatibility with other addons, and a fix to the dupe glitch.
Let's start with the new ore: multidimensional rock.
This rock has dimensional properties and is found through all main dimensions.

This rock can be made into armor which serves as a slightly better material than iron, and the tools are just above iron as well.
While the armor isn't special, the tools (except for the hoe) are.
They can be randomly enchanting by using them once. 

You can also take it and make a multidimensional jukebox with it. It plays the 3 new discs (by me) that are dropped from the bosses now. 

Then there's the new boss who you now must kill near these strange monoliths to gain a portal key to even gain access to the new dimensions now. 

Btw, they can drop their scythe or a music disc by chance. 

The scythe is quite durable and deals 6 damage.

If you take multidimensional rock and brew it with a mundane potion, you will gain the multidimensional potion. An odd potion that grants immunity to negative effects and agility but gives an interdimensional high effect that is uncurable for an entire minute.

V4.5 UPDATE:
There is now a few new things that have been added and fixes as well. The multidimenional tool recipes were mixed up, so that was fixed.

If you defeat the cavern spider, you can get its chittin, which is used to make a potion. Same with the death worm: if you kill it, you gain some saliva, which can also be made for a new potion. (remember to use mundane potions for brewing these) 

You can now find Sky's cow in the skylands. This cow is feedable with melons and grants water buckets. Quite strange...

On top of all that, 3 new discs were added. 

v5.5 UPDATE:
This was an update meant to fix a few bugs and mainly tackle the issue of compatibility with newer versions. Be warned it could still be buggy,

v6:

3 new discs. A new winter dimension. A new potion. Made compatible for 1.18. No spoilers.

v6.1:
The addon had a glitched winter and cave dimension, so an emergent fix was made to it. Here is the Algid portal for those who did not know how to get to the Algids (winter dimension, and keep in mind the ignition is the snowball item): 

v7:

Expansions and support for 1.18.30 are here, a video showcases it good enough: