Hope it's not too late for suggestions. I came up blank at first, but during a brainstorming session, I suddenly got a crapton of ideas for new blocks.
Bombs, beetles and aliens corresponding to the other types of guns and lasers:
Things like teleport aliens, use bombs or damage beetles.
Cute animal:
Completely stationary while onscreen. Using it either puts it to sleep or wakes it up. While asleep, it's completely inactive. When awake and offscreen for a few seconds, it will target the nearest offscreen animal, then instantly teleport there and replace it while making a disturbing sound reminiscent of the animal being eaten. It will not do so again before first appearing onscreen again. (In case it wasn't clear, the idea is that it pretends to be cute and harmless towards you, yet is actually a terrifying monster.) This allows it to be used to get rid of animals in locations that would otherwise be difficult, if not impossible to reach. But it can also serve as an obstacle that you have to keep an eye on so it doesn't kill animals you need.
Mirror:
Is shaped like this:
x
x x
x x x
x x x x
x x x x x
When a laser hits its diagonal edge, it will be reflected in a 90 degree angle. Its other two edges are non-reflective and vulnerable to lasers. Using it flips it horizontally.
Thief animal type 1:
Walks straight ahead along the ground and changes direction when something blocks its path. When a block appears in front of this type, it will shoot straight towards it, ignoring gravity in the process. It will then pick the block up. Here's how the process works:
F=Floor
T=Thief
B=Block
A= Air
Step 1:
A A A A A A A
T A A A A A B
F F F A A F F
Step 2:
A A A A A A A
A A A A A T B
F F F A A F F
Step 3:
A A A A A A B
A A A A A T A
F F F A A F F
Step 4:
A A A A A B A
A A A A A T A
F F F A A F F
It will then continue walking while carrying the block for a few seconds, then use it once before dropping it below itself. When used, it will shoot straight ahead (potentially carrying you with itself). If it collides with a block, it will pick it up, then use it after a few seconds as usual. When dropping the block, it will free itself from your grip and put the block into your hands instead. On the other hand, if you put the thief in your inventory while it's carrying a block, the block in question will not travel to the inventory with it. This allows it you to release blocks from its grip before it uses them. It refuses to pick up blocks with a price tag, and won't shoot towards them unless used.
Thief animal type 2:
When you bring a block out of your inventory while it's onscreen, it will get the block instead, carry it for a few seconds, then use and drop it. If multiple type 2 thieves are onscreen simultaneously, they will all get an instance of the block, thus cloning it. Aside from that, it behaves mostly like a type one thief. The only exception is that it refuses to pick up blocks lying around, and won't shoot towards them unless used.
Thief animal type 3:
Also behaves mostly like a type 1 thief. While it's onscreen, it has access to your money. If it spots a block with a price it can afford, it will shoot towards it and pick it up, spending your money in the process. It will then use and drop the block as usual. It refuses to pick up blocks that are free or too expensive, and won't shoot towards them unless used.
Nuke:
Works similarly to its FiNCK equivalent in that it destroys everything in the current room when used. As such, you should handle it with particular care if you bring it into the save room. Like other bombs, it can also be given a timer, which starts ticking upon usage and can be frozen by putting it into your inventory.
Holy block:
Removes the current room's curse when used and destroys itself in the process. This is undone upon your death.
Vacuum beetle:
When you spend too much time near it, it'll destroy itself and pull anything in a fairly large radius towards itself with great force. Aside from that, it's just like a regular beetle.
Magnet gun / alien:
Its bullets deal no damage, but pull any blocks they hit towards the direction they came from. Makes it somewhat of an antithesis to the weak gun.
Spider:
Its legs can point in one of four directions. They will stick to solid things and allow it to walk along them. When encountering an unmovable obstacle in its path, its legs will change direction to cling onto it. Using it changes its legs' direction as well. While its legs are clinging onto something, it completely ignores any pressure exerted on it. As such, it can easily push blocks lying in its way. It can even carry you while climbing walls or the ceiling if you're holding onto it. It's considered unclean, so if it comes in contact with an altar, said altar will be defiled and can no longer be used for sacrifices. This could allow for puzzles where you have to either prevent a spider from reaching the altar, or blast through the skull blocks with heavy weaponry.
Clock:
Using it toggles its aura on and off. Anything within its aura has its speed altered until leaving it. This includes yourself. How much they speed things up or slow them down can be freely adjusted in the editor. The speed at which a clock's hands move shows how it will alter time.
Time gun:
Adjusts the speed of anything it hits, including yourself. This speed alteration effect doesn't wear off by itself and can't be removed by hitting said thing again. However, it will disappear if the thing in question is moved to a different room. It can also be overridden with a different speed alteration effect when hit by a different time gun or entering a clock's aura, though this is only temporary in the latter case. It has a small clock attached to it whose hands indicate how it will alter time.
Human ice box:
Can slide like a regular ice box, but can't absorb blocks. If you stand on its lid for a few seconds, you'll be sucked into it. Anything you're carrying is left outside. In this state, you can't carry blocks or enter doors. You can however make the box slide to the left or right, or make it jump. Bringing blocks out of the inventory will place them on top of the box. Also, since the box is just as small as a regular block, it'll allow you to fit go through small crevices. If the box is destroyed, then so are you. Attempting to teleport the box will destroy it. Short of dying, the only way to escape the box is to find a way to use it. It can also absorb NPCs, though only one person at a time.
Golden heart:
When used, heals you and gives your health bar an extra heart, allowing you to take two additional hits. You can increase your health even further by using multiple golden hearts. However, the effect disappears upon leaving the room. NPCs and thieves can also use it to boost their own health. Since thieves, like all animals, start off with only one HP, a golden heart boosts their health to three, not four.
Heal gun / alien:
Heals you, NPCs or thieves that have boosted their health. However, whereas hearts (both pink and golden) fully restore their HP, the heal gun / alien only heals them by one HP per shot.
The following things aren't technically blocks for the most part, but other gameplay elements I came up with.
Heal laser:
Fully heals you, NPCs or thieves that have boosted their health.
Time laser:
Basically like the time gun and alien, only as a laser.
Strong curse:
Is indicated by skulls with glowing eyes. Completely denies you all access to your inventory. Using a holy block in such a room merely downgrades it to a regular curse.
No inventory door:
Can only be opened while you have no blocks in your inventory.
"$ALL" price tag:
Requires you to spend the exact amount of coins you currently have to pick up the block it's attached to. In combination with type 3 thieves, it can be used to forcefully empty your wallet.
Lock:
Can be applied to any block, and will prevent it from being used. By pressing a certain button, all blocks in the current room (including those in the inventory) will be unlocked. Furthermore, blocks are automatically used during the unlocking process (unless they're in the inventory).
Tether:
An immaterial connection between a block and either another block or a tile. Its maximum length is freely adjustable in the editor. Once the distance between its two ends exceeds said length, it permanently disappears. This also happens if a block it's connected to is destroyed. Furthermore, there are buttons that can sever tethers as well. Tethered blocks can't be put into the inventory, absorbed by a box, be teleported or leave the current room. When a tether is severed, any locked blocks connected to it are unlocked.
Rubber tether:
Works like a regular tether, except when its maximum length is exceeded. Doing so doesn't sever the tether, but instead causes it to pull any blocks connected to it towards its other end. The pressure increases at greater distances, making it comparable to a bungee cord. It can only be severed by either destroying one of its blocks or pushing a corresponding button.
Lock gun / alien / laser:
Applies locks to any blocks it hits. When it hits you, all blocks in your inventory are locked. Has no effect on blocks that are already locked.
Tether gun / alien / laser:
After hitting two things, they will be tethered together. The type and length of the tethers it creates can be adjusted in the editor. If a tether is attached to it, it will be reapplied to whatever the tether gun / alien / laser hits next. Can also apply tethers to humans and tiles, not just blocks.
Usage limit:
Can be applied to any block. Decreases by 1 every time its block is used. Once it hits zero, the block is destroyed.
Glass:
Another property any blocks can have. Causes them to be destroyed when colliding with too much force, similar to the Glass Ball in WaDF.
Glass gun / alien / laser:
Turns any blocks it hits into glass. When hitting you, it affects all blocks in your inventory.
Heavy block:
Like its FiNCK equivalent, it can't be thrown very high and reduces your speed and jumping height when carried. However, it also greatly accelerates when falling. Once it has reached its maximum speed, it will inflict the same kind of damage bombs and the red gun and alien deal on anything it falls on. This means it will destroy fragile blocks and bricks, and use sturdier blocks.
Generator:
Creates a certain block. Once said block is destroyed, removed from the room or put in the inventory, it'll create another block just like it.
Sound sensitivity:
When this property is applied to an inactive laser, it will be active as long as certain BGM sounds are playing. It's the opposite for an active laser. Sound sensitive generators don't work normally and instead spit out a new block every time the sounds play. Blocks with this property can't be used through normal means and will instead use themselves whenever the sounds play.
Finally, I'd suggest a handful of changes to existing parts of the game.
Two textual changes regarding beetles:
IMO, the game doesn't do a very good job explaining that as long as you're fast enough, you can prevent beetles from transforming by picking them up and using them. I've already seen two YouTubers who didn't know about this until I told them. I think this is because the book about beetles only mentions the possibility of using them from afar, and because when carrying them, the game tells you using them would transform them, which is exactly what using them prevents from happening. I'd suggest rewriting the book and changing their use term to "Disarm".
Using books from afar:
Right now, books do nothing when hit by a use gun or laser. Not only is this illogical, but allowing them to be read from afar could allow for clever puzzles where you have to find a way to use an unreachable book in order to get a message telling you how to progress.
Voice clips for animals other than cubes:
I really liked how cubes make all sorts of weird noises, so I found it a bit underwhelming that the other animals don't do anything like this. Especially since cubes aren't all that prominent in the later parts of the game.