I was reading the MLS thread and I realized that over the years a number of ideas appeared very bright as they were launched only to be abandoned only few years later. Besides the MLS, what are the ideas you remember that appear and went out of sight some time later without any commercial application?
I rather think that the OP was asking about "ideas" or "concepts" that didn't work out or catch on, eg piggy-back, wheel-less deck landings or prone plots rather than merely a list of aircraft types that didn't sell, a category that must be way longer than the list of those that did...