The stairs in front of the door are quite easier to understand, when we can reverse engineer the solution!
The side of each items were actually pointing at the tendencies, pure white when the item is to the left and pure black when the item is on the right.
The canteen on the first step was for Biorr in 1-3, pure white (left side).
The pot on the 4th step was for the Filthy Man in 2-1, pure black (right side).
And the bag (location still unknown) being on the 5th step and to the left side, points out toward the 10th coin, 5-x pure white.

If we cannot find the 10th coin anywhere in World 5, I suggest that we go back to the door and check the sack after getting the 9th coin.

But there's something else. There is seemingly 6 steps on those stairs, and the number 6 keeps coming up (6 steps, 6 ornaments on the door, 6 people tweeted at by BluePoint...). If we get the 10th coin, and nothing happens, I think we have to look in the Nexus, and probably in pure white and pure black, if not for the World tendency then maybe in character tendency.