Here are LOTS of ideas I thought.

1. Reverse lockholes: A door with pink lockholes that requires POSITIVE keys to open, but instead of SUBTRACTING the key of the following color it ADDS the key of the following color.
   1. Its negative variation is a door with cyan lockholes, which requires NEGATIVE keys and SUBTRACTS keys.
2. Convert keys: A key object that convert all the key of one color to another. For example, picking a pink -> cyan Convert key while you have 2 pink keys and 3 cyan keys will make it become 0 pink keys and 5 cyan keys.
3. Keepable keys and doors: A key or door that can be not interacted if a certain button is pressed. If it is a key the player can pass through without collecting it, while if it is a door it will not be opened and can blocks player.
4.  Variable keys and doors: A key or door that can changes its key amount in a certain situation, or changes its color, or even appearing and disappearing, and many more.
   1. Its possibility is endless!
5. Tricolored doors and doors of more colors: Similar to Bicolored doors, it will modify the key count of  ALL the colors in its outer border.
6. Simple doors: A door that can only be opened if the key count of the following color is not a complex number. For example, a Simple door can be opened with 0, 3, -2, 5i or -1i keys, but not a complex number of keys like 1+2i.
   1. Its variation is Complex door, which can only be opened by a complex number of keys.
7. Unstable Glitch keys: A new key type that can changes its color even after collected, and it will treads as the color of the last door opened when you opening a door with that color. When you have both Unstable Glitch Keys and the normal Keys of a color, you always spends the Unstable Glitch Keys of that sign first down to 0, then the normal Keys with the difference between the door cost and the Unstable Glitch Keys you spend if you do not have enough Unstable Glitch Keys. For example, if you have 2 Unstable Glitch Keys mimic as Green and 3 Green Keys, opening a Green 4 door would cost 2 Unstable Glitch Keys and 2 Green Keys, and opening the same door with -1 Unstable Glitch Keys and 4 Green Keys would cost you 4 Green Keys without costing your Unstable Glitch Keys, since it only costs the Unstable Glitch Keys if it have the same sign (which also means it does not simply adding the -1 and 4 to become 3 that would made the door can not be open.). The Unstable Glitch keys can not mimic to Pure, and when the normal Glitch mimic is Pure it will mimic the "Glitch" color instead. Opening a Blank Door that have the color the Unstable Glitch Keys mimics require the Unstable Glitch Keys and the normal Keys summed to be 0 to be opened (The AND logic works too but I think the sum logic is better.). Opening a Blast Door only requires you have either the sign of your normal Keys or Unstable Glitch keys to be that door(Since the Blast Doors only checks the sign of it), which costs the number of your normal Keys of that sign if your normal Key meets its sign, and the number of your Unstable Glitch keys if it does, and the sum of those two if both meets. Opening an All door simply costs the sum of those two, ever if those two sums 0 unless both of them are 0(Since you only need "some" keys to open it, and it is not simple adding for the Unstable Glitch keys' logic). Notably the Unstable Glitch keys only replace the normal keys when opening doors. It can not be used as a Master Key when its mimic is Master, nor the Red/Green/Blue/Brown aura effects.
   1. Unstable Glitch Doors can be added too. Unlike Unstable Glitch keys, it does not changes its color, and it can be opened with Unstable Glitch keys only regardless of its mimic color. Normal Glitch mimic can not be Unstable Glitch, and opening a Unstable Glitch Door will turn the normal Glitch and the Unstable Glitch mimic to the normal "Glitch" color instead, regardless of the current Unstable Glitch mimic.
8. Dispel aura effect: A weird aura effect that make the affected doors cost nothing after opened. The base cost of the door itself still need to be afford before it can be opened.
9. EDIT: 2024/07/13 Glass Doors: A door variant that has transparent looking, when you met its requirement, you break it as normal and cost your keys normally when you touch it, but if you do not met its requirement, you will break it without costing anything. For example, touching a Glass Orange 6 door when you have 6 or more Orange keys will open it normally and cost you 6 Orange keys, but if you have 5 or less Orange keys you will break it too and let your Orange keys unmodified.
   1. Those doors use the glass-breaking sound exists in I Wanna KeyPick 100, played when you open a door that costs negative number of keys.
10. Silver Keys & Doors: A third key and door type that have glowing animation as Master (Gold) and Pure (Platinum) keys. The Silver Doors can be opened by Master Key charges but can not be cursed, as Master Doors can be cursed but can not be opened by Master Key charges, and Pure Doors can not become both.
11. EDIT: 2025/01/11 Swap doors: A door with two colors similar to a Bicolored door, it requires you to have enough keys of the Lock color to be opened and when you open one, you cost the keys of the Lock color normally and gain the keys of same amount of the Swap color.
    1. For example, a Swap 6 door with Green as the Lock color and Red as the Swap color requires 6 Green keys to be opened, and it will cost 6 Green keys and gain 6 Red keys when opened.
12. EDIT: 2025/01/17 Flip Keys: A new Key type that have an active use similar to Master Keys, when you use one on a door, its copies count will be signflipped. Negative Flip Keys does nothing but still get consumed. Imaginal Flip Key multiply the copies by -i and Negative Imaginal Flip Key multiply the copies by i. Doors with Flip or Pure parts are immune to it but Master is not immune to it.
    1. For example, on a door with 1+2i copies, using a Positive Real, Negative Real, Positive Imaginal and Negative Imaginal Flip Key on it will make it become -1-2i copies, 1+2i copies, 2-1i copies and -2+1i copies separately
13. EDIT: 2025/07/18 Mold Keys: A new key type that have an activated use like Master Keys, it works almost exactly like Master Keys but it works on Key objects instead of Doors. Positive Mold Keys destroys a Key object without collecting it and Negative Mold Keys copies a Key object. When you copied an Key object or collected an Key while still have remaining copies, it will be disabled until you walk off like the Infinity Keys does. Mold and Pure Key objects and Infinity Keys are immune to it, including using an Imaginal Mold Key on an Real Infinity Key object.
    1. This idea looks silly but I found it is interesting when you can copy the Signfilp and Star/Unstar Keys.
14. Fractional Master Keys: An abandoned idea in Lockpick Museum, now I reused it. You may collect a Master Key that have float numbers like 0.5, and when the count of your Master Keys is not an integer, you will have the weird effect on the Master Keys exactly like the museum said, which when you have the Positive count of Master Keys, using it will works as Positive Master Key and subtracts 1 Master Key from the count, which may let it down to Negative, and vice versa for the Negative float Master Keys. Basically, it allows you to destroying and copying a door alternately without running out it. But it can still be worth a lot on the puzzle solving by simply adding the Blank Doors that forces you to collect another Float Master Key to cancel it off.
    1. Float Master Doors is also worth a thing in this idea, which have the Float number of Master locks on the door that would put the count of your Master Keys to the Float numbers too and it can be copied to making it able to use one to put the keys to float and using the same door to get back to integer.
    2. Beware the Blast Master Doors which may put the keys of the other colors to Float too, though. Even the Master Doors can be Float, the other normal Doors and Key objects would be very rarely to be Float in my idea.
    3. For the Partial Blast Doors in the MathCookie's post, It would become not able to open any Partial Blast Doors that have an integer denominator when the count of your keys is Float, since you can only open them when the count of your Keys can be divided, and none of the Floats numbers can be divided by an integer. Though it is still able to open them if the denominator is Float too and the count can be divided, such as opening a X/1.5 door when you have 4.5 keys. Normal Blast Doors can be opened with Float numbers normally but a Partial Blast Door with X/1 can not, which would still costs the same amount of Keys like the normal Blast Door does when you can open them. 

Also see: 

• I have several ideas for door properties (spoilers for pretty much the whole game) - I Wanna Lockpick community - itch.io
• Idea: Partial Doors - I Wanna Lockpick community - itch.io
• Idea: Switch Keys/Doors - I Wanna Lockpick community - itch.io
• Idea: Absorbtion keys - I Wanna Lockpick community - itch.io
• Idea: Multiplying Keys - I Wanna Lockpick community - itch.io
  
• What if all 11 worlds had a "return" variant? (A list of ideas) - I Wanna Lockpick community - itch.io

 More ideas for the PILE:

Shadow Doors/Ghost Doors.

A incorporeal transluscent door type that doesn't have collision. (Basically being walkthrough if you don't meet the condition and dissapearing after the keys are spent. This could be paired with any other door type, and could lead to intresting dynamics.

Edit: holy crap I predicted the name. dang. Well this is now unoriginal. Big sad :( 

Skeleton Keys. 

A master-esque key that turns the door it touches into a ghost door. Negative versions turn a door back.

Badges.

Gold, pure, and skeleton stickers that are placed on doors, functioning as a immunity to those door types. Essentially a extra outside color without the cost swap.

Minilocks.

These would be doors that effectively would function as keys, being put in a 1-10 door inventory when picked up. They could be stacked and would jam you from using master keys, because they would automatically trigger when their conditions are met, being resolved from top to bottom, left to right. I hope you aren't carrying any brown keys. 

Quantum Doors.

Doors with a dotted gray line between them. When one of them disappears or is cloned, the other is as well. Effectively being a minor Linked Door. When one is salvaged, both of them appear in the same spot they were relative to each other. A Quantum Door would also count as one door for the purpose of all auras.

Updated a new idea, thank for Balatro, a rouge-like card game.

Updated an idea, Swap doors. That is what I guessed what do they do when I first saw the Bicolored Doors.