So while i was playing around on the aurora isles i thought that something was missing...same happend in a poison like construct area...
(Note that some of these MAY have been already posted on some thread but at least they are renewed so that it is easier to find them)

So my suggestions would be adding:

Water:"Again that water!" would be the reaction that the knights would express becoz:
Water would slow you down by 15% noted by a effect called "Wet!"
Water would slow you further the deeper you went in going like this 15% - 20% - 25% and would finnaly stop at 30%
A good thing about water is that enemies are affected too,Constructs wont enter water becoz they would rust and Explode...so they wont enter!

Lava:Entering it would inflict you with fire + wet!
The lava wouldnt damage you but instead the fire effect that would be given to you!
Enemies including Slag walkers + Fire slimes would get healed in it...

New levels and stages:

Aurora island - Watery Dangers
It would consist of aurora islands connected with Iron bridges that have ponds of water everywhere...
Enemies that would spawn would be gremlins,slimes,Mini enemies,Lumbers(not near water but instead on those mini arenas) and undead(if the level is undead based)

Flaming Forest -Burning Witness
It would be a flat area with bumps and tress everywhere...there would also be a river of lava and there would be around 1-2 arenas in it!
Monsters that would spawn could include Slag walkers,Oilers,Big oilers,Slime turrets(fire version) and Fire lumbers!

Deadlight Plains - Haunting lands
Would consist of A town connected to a graveyard like area.. the town would be a friendly area with some knights and a special vendor,and the graveyard would be the hostile area...There would be more phantom spawns and more zombie spawns..also in one arena you would have to fight a trojan + all 6 kinds of zombies in 1 arena!

Boss area: The walker (my boss- you can find the boss thread somewhere)

Would consist of 3 areas!
1: Burning factory - Factory like area with broken bridges here and there...would have some puzzles and 1 arena before going to.....
2: Shocking Searchlights:A area that has a lot of those Red searchligts that would shoot the rockets(collector mission) it would have a arena at the start and 1 puzzle at the end that leads you to 3 green and 3 red boxes and the elevator to....
3: The Project Base : a Round arena(Round doesnt exsist) that would at the start have the walker shut down in front of the Party button..
When stepped on the walker would turn on and for more info chechk The walker boss thread!

Thats my suggestions for more enviromental stuff + The gloaming wild woods need a hidden slenderman? right?!