Wikibuilder would create a knowledge base covering the design and construction of the built environment, in its entirety, in all languages.
From building Skyscrapers to Japanese cabinet making to electrical engineering to running your own cooperative. It would cover absolutely everything to do with the built environment.
Much, if not most, of the information could be presented as text (and could start off as just that), but text presents profound and obvious limits for describing the built environment, so much of the information would be pictorial. With the drawing expertise of designers and draughtspeople around the world who are intimate with CAD (computer aided draughting) there should be no shortage of people to get the ball rolling.
There are currently efforts going on in the graphics and architecture industries to standardise pictorial file formats (see links below) and the main topic of initial discussion on Wikibuilder may well revolve around the kind of file formats to use for presenting and distributing sketch, 2D, 3D and other kinds of pictorial information; special code may need to be introduced into MediaWiki to deal with presenting/previewing these up-and-coming 2D and 3D file formats. Carrying on from this might be discussion on graphical styles and style standardisation.
There are a few open-content repository-type websites around for sharing building details, et cetera, but most of them are woeful and narrowly focused on CAD technicians looking for details to use at work. There're a myriad of websites dotted around the internet offering information on design but this is extremely fragmented.
Ever since we as a species started manipulating our environment we have built a vast knowledge of designing and constructing the built environment. This knowledge belongs to everyone, but much of it is locked away in people's heads with no easy way to share such information except for books (usually expensive ones) and apprenticeship. A wiki focused on the built environment could help unlock this knowledge and make it accessible to vast numbers of people (builders, designers, inventors, diy'ers, knowledge lovers) who could put it to use and continue to build on it in a open way. It would be especially valuable for cost estimators, whose task requires a comprehensive overview of the construction process. A wiki could also help the push for open standards in the presentation of pictorial information via the internet.
This information could be especially valuable in developing nations where a little knowledge could provide profound effects -- free instructions and diagrams (in many languages) for everything from irrigation and chicken coops to specialized tool design to car and computer repair could be an invaluable resource for families and communities, or for aid workers helping them. More and more communities have some sort of Internet access to read and refine these plans for themselves, and building these resources collaboratively on the Internet is the first step toward making them available in print as well.