Are there any hard rules aside from the few examples given and "use your imagination" to build your own wondrous items?
Yes, RAW is existing for wondrous item creation.
For general information regarding item creation read the "Magic Item Creation" section. After that the "Creating Wondrous Items" section of the pdfsrd may answer all your questions. This article also contains a link to a table called "Estimating Magic Item Gold Piece Values" for estimation of prices in respect to the modifiers used and so on.