The 1956 Mercury XM Turnpike Cruiser concept car fell somewhere between the first and third categories just mentioned. It was futuristic, but lent both its name and some styling elements to a forthcoming 1957 Mercury model. Links with information about and images of the show car are here and here.
For some reason, photographs of the XM Turnpike Cruiser are hard to find on the internet. Some of the images posted below, for example, seem to be scanned from magazines or other publications. The links feature these and other images including some showing a specially-built display truck-trailer combination that toured the USA showing off the XM Turnpike Cruiser. My father and I visited it when it stopped off in Seattle.