Whole campaign.
I think ends of levels should all have a tunnel slide at the end. Imo it plays very nicely with gish being a goo, having to slide in the sewers etc.

So, there are five worlds and 34 levels in total.

Worlds contemplations.

They should obviously be harder and harder as the game progresses.

New worlds should bring in entire concepts like ropes, moving blocks etc.

World 1. The player is new, so easy levels, no traps, maybe obviously placed secrets. Sliding blocks. Mobs should be present because of the boss fight at the end.
World 2. Introducing some parkour, maybe puzzles, some hazards like spikes and water. Throwing blocks. No lava yet. (Just like in the original game)
World 3. Harder parkour, maybe with cyclic timings. Lava. Traps. 
World 4. Long levels with hard parkour. Hoards of enemies.
World 5. Vertical movement. Gimmicky levels, should be especially clever.


World 1.
Level 1. Tutorial with helper areas (!!).
"Press the '/s' key to slide down the hole."
"Switches and buttons sometimes open doors."
"Press the '/s' key to stick to the wall.  Press the '/s' key to begin climbing"
"Press the '/s' key to jump. Gish jumps best when he is" the most compressed, bouncing up and down will help you get higher and higher."
"Press the '/s' key to break these blocks."
"Press the '/s' key to stick to the wall.  Press the '/s' and '/s' keys to climb. If you are stuck to the floor, quickly release and then press the stick key"
Introduction to physics objects (boxes)


Level 2.
Introduction to switches
Introduction to nibblers
Very low platforms
Interaction with physics objects (rolling a ball into a pit)

Level 3.
Introduction to water
Introduction to spikes
Zombie enemy intro
Moving block onto a button

TODO: Warp secret to three locations

Level 4.
Chains
More wall climbing
Slidey trapdoor
Platform which shoots up

TODO: Warpzone to 4-3
1-up in a secret

Level 5
Sliding wall climbing
Taking block from the bottom of the pool
Enemy trap
Multistage buttons

Level 6
boss battle

Level 7
Cart ride

Cool swing
Pulley door


World 2
Level 1
Center spinning block (low amplitude)
Edge fixed bridge
Rope block
Brittle floor

Level 2
A couple of mobs (I'll skip them probably)
Strong floor
Throwing a block (simple)
Water
Rope platform jumping (not here, surely, player doesn't know ropes at all yet) - maybe chains here
Moving blocks with gaps to hide

Level 3
Water
Mobs
Pulley door
Taking block from the bottom of the pool - Done
Huge underwater section with coins near spikes - Done

Level 4
Multistage buttons again
Cool trick with jumping block and then taking unused cube to a later stage
Elevator

Level 5
Center rotating block (full 360 spin)
Cycle platforms
Rope platforms attached to them
A lot of them with climbing up
Rope blocks across
Sliding split

Level 6
Dark boss battle

Level 7
Chains riding Cart
Lava at the bottom


World 3
Level 1



Carousel

Companion cube level:
jump on the cube to get higher
in the end throw it into lava :(