a few silly/mildly goofy TOTALLY SERIOUS suggestions of mine, totally not way to late to be implemented.
treadmill - pushes the player character in one direction really fast, and the direction can be changed with a (world 8 gimmick)
endless hallway - a really really long level were one must simply run down an exceedingly long hallway to grab a key and then turn around and run all the way back to win the level
quantum keys - you don't know how many keys you have picked up, and you also don't know how many are needed to unlock each door until you hover you mouse over it (observing it)
time dependent keys - the amount of keys picked up is based off of the time of day in real life.
bucket of paint - turn all of your current keys into another colour, but after a while the paint dry's off and changes into a different colour, also changes the colour of the player character, they paint can be washed off by going into water.
poisonous gas - whenever you step into the gas you begin losing random keys at a rapid pace.
brick - is that door in your way? No worries! hurl a brick at that abstracted rectangular obstacle to break it instantly! just like in total chaos the game. [one use per brick due to fragility issues [just like in total chaos the game]].
Nokia phone - acts the same as a brick except can be re-used forever.
diamond encrusted drill - the first puzzle of world 8 too hard? simply drill through the wall and get that door! totally balanced.
paper keys - act just like regular keys, but if go into water they become...
soggy paper keys - you have to be moving slowly or else you will tear the keys apart, if you have any (world 5 gimmick) keys, after about 10 seconds they will dry out and become...
warped paper keys - the result of drying off your soggy paper keys