I've added a new door today that has been hotly requested: the Fee Door.
With a Fee Door, you can charge other players gold to open your door. You configure the amount with /feedoor fee <amt>. As the owner, you get to enter for free.
When another user presses E to open the door, they see the door's cost. They can pay the cost and open the door by typing /feedoor open, or just /fdo
The door comes in both left handled and right handled varieties -- but since they charge independently, I don't recommend using them in double-door pairs.
The door also acts as a temporary safe -- gold is deposited into the door, and when the owner opens it with E they can see how much gold it contains as it is then deposited into their main account.
Thanks to Bruno, Meep, and everyone else who voted and commented on this suggestion in the suggestion box.