Well with Software why not try to strike a deal with iGuidance people or since you already have Microsoft products, then why not sell standalone Streets and Trips or Mappoint. In my opinion the software section is seriously lacking.
Maybe some 1.8" harddrives (like what is found in ipods) with mating adapters. Good for those times when there is no room for a full HD. COuld be an option to those who dont know that are thinking to go the CF Card route. Speaking of which maybe a CF Card and IDE adapter combo.
Those lilliput replacement cables like armen has, lots of people look for those along with replacement TS overlays.
More will come to me later.