From the update:
The next two stretch goals after we unlock the Interior Wall will be the Cargo Hold and Corridor Two Side Room. The Cargo Hold is a large room (approximately 8"x8") that has doors at two ends and is two stories tall. Included will be removable catwalks along the two walls that don't have doors. The picture I have included here is of a work in progress model of the Cargo Hold. The final piece will not differ substantially from what is shown here but will include more detail touches. The Cargo Hold will be an Advanced Stretch Goal.
The next stretch goal after the Cargo Hold is the Corridor Two Side Room and will be a Basic Stretch Goal. This piece is a modification of the Corridor Side Room that we have already unlocked and adds another room "right across the hall".