The Philosophy of Nothing (Skyblock)

Make a glass platform as big as you can and wait for the night. Mobs don't spawn on glass. At night, stars will start falling and you can pick them up and make blocks with them. 
Make a mob spawner until you get enough iron (better quests gives you some iron to ease grinding), then make a cauldron, then a bucket.
Put the cauldron anywhere to get some water (after some rain). To get lava, you have to hop into the end portal below the starting bedrock, and use the Philosopher's Stone on the obsidian to make into lava. The dragon will die upon entering the End. Now you have a cobblestone generator.
You can get wood from the Underworld Dimension from LightningCraft, but it will be a while.

From here, you can do whatever you want. You can grind for iron to turn into diamonds to make a pickaxe to mine obsidian to get to the Nether. Or you can take lava from the End one bucket at a time and convert it back to obsidian. It is up to you to figure out.

Underworld Tree guarded by Demon Soldiers:

Another Underworld Tree guarded by Cursed Armors:

- Armors have almost 0 durability. This encourages the player to actually be careful around mobs and not just armor their way around.
- Vanilla shields are disabled. Hopefully I got all the ways. Again, it encourages the player to be careful.
- Most LightningCraft and ProjectE tools can't be crafted, since they kind of break balance. Avaritia is also completely disabled except for Neutronium.
- Guardian Skeletons from LightningCraft drop sapphires. This makes them renewable without the designated quest. Edit: now Sapphire Ores also spawn in the Underworld, but sometimes not the first time you visit a Stone area.
- Scaling Health is on, therefore the world will get harder as time passes. However, this only affects the Underworld and the Arcane World's Dungeon DImension

- I disabled all wood except for Underwood, it's possible to get leaves because BlockReplacer sometimes gets bugged, but I disabled both sapling growth and dropping.

- I added ores to the Underworld, Sapphire and Rock Candy, but sometimes the BlockReplacer mod doesn't work the first time you generate a new chunk and have to leave the dimension then come back for the ores to actually spawn.

- The quarry can mine mycelium (without silk touch) if the player uses a biome marker with a Mushroom biome on it

- Demon Soldiers no longer spawn in the Nether, but recipes were adjusted so you can still reach the Underworld no problem (with still 2 ways to get Demonstone and Underworld Charge)