Here is a design document I started for a game I will never get around to making. Thought it might spur some ideas for some folks here. It is an MS-Word doc.
http://www.scottshaver2000.com/temp/scl.doc
A bit from the overview section:
You work for the Space Construction League (SCL) as a contract construction pilot. The term pilot is a bit of a misnomer; actually you are sitting in a control ship where you remotely control a construction pod. Your job is to dock the construction pod to project modules dropped off in space, by automated space freighters. Once you have docked with a module you have to take it to the build location for the project and connect it to the project in the location indicated by the blueprint you have been provided.
While you are doing this you also have to protect the project and modules from being hit by asteroids that are in the area. Once the project is complete it can protect itself. If the project is hit by an asteroid modules maybe damaged, destroyed or disconnected from each other. When a module is destroyed a replacement will be needed. When a module is disconnected you must put it back in place.
As you are building the project you will need to place orders for the project components you need. When an order is placed one of the automated space freighters will deliver the module. The space freighters are buggy and often drop the project modules off in places far away from the project build location. You will have to go get them and while you are away your project is vulnerable. You shouldn’t leave a module floating in space for long by itself because the space pirates will come and steal it. If this happens you have to order another one.
You want to build the project within, or under, the budget supplied for construction for the project. When modules are destroyed, damaged or stolen they must be replaced or repaired using the money from the project budget. The more time it takes you to complete construction the more the project costs.