This is the first idea that I came up with. A puzzle/platformer.

The entire level is visible on the screen (2D). All the collectibles are clearly visible and you can trace what path to take to find them. You start with one item in your inventory; an arrow pointing right. The controls are the arrow keys. But, since you only have a right arrow, you can only move to the right. The other keys are the scattered around the level. Some of the controls are in hard to reach places. Impossible to reach without a specific control such as space (jump). You have to figure out how to get all the controls and reach the end of the game where a locked door awaits you. The key holes are in the shape of arrow keys as well.