The Future: Construction
Doors: Since the items are complicated, these will be too. How it is opened, which direction it opens, locks, material, size, condition, secret doors (probably put these in at the same time I do trap mechanisms)...
Windows: Like doors, these can get complicated.
Buildings and other Structures: Good stuff, but incredibly messy without some abstraction. How does building construction work? If walls have to put up board by board with individual nails, not only could it get tedious, but the game would have to decide when the construction becomes a wall. There is also the matter of some architecture: archways, reinforcements, pillars, beams, doors, windows, ceilings, floors, etc. The Z coordinate will allow stairs and multiple floors.
Making items: To what extent does the game support user items? Can the user invent whole new classes of items, or just make items based on the existing master item list (an item is a set of restrictions - for example, a "short sword" is defined to be a two component item with the restriction that one component be a simple blade made from a rigid material. The blade length must fall within certain parameters. There are also restrictions on the hilt component; it can't be so long that the sword looks more like a spear). It would be straightforward to allow the user to make any sort of construction they like, but then it might be difficult for the game to determine what can be done with it and what skills apply, etc. In the end, it will probably be a compromise, with some reasonable restrictions on the player while still allowing a great deal of freedom. Each step of the item production process will use different skills, and failure at different stages will lead to different results. For instance, a sword blade might be very sharp, yet attached to the hilt very loosely. This can lead to rumors critiquing craftspeople in subtle ways. There might be many clues in an item's make that tell who made it or for whom it was made. Maybe a charm over an item can somehow indicate where the item was built (possibly destroying it). Certain quirky creatures might forge powerful items in lava, etc.
Collateral Products: Various forms of construction produce stone dust, saw dust, wood chips, etc.