txoilgas:
Again the whole purpose is to cover those that fall through the gaps of the larger broad brush categories.
As I think I said initially, designing a category to fill gaps is immensely difficult. Fitting a category into ONE gap is hard enough, but to to build one for multiple gaps of different shapes and sizes is formidable. And, I think that's the problem. California has one way of listing and managing historical sites of various kinds, Texas another, Colorado yet a different scheme, and then there are cities, towns, counties, entire countries, prefectures, cantons, provinces, etc.
Then you have different categories handling these in different ways, from state to state and country to country, with some being very broad and inclusive and others being narrow and restrictive. Additionally there are categories that handle segments or special types of historic sites from the two NRHP categories, to Engineering Landmarks. These all interlock and overlap in various ways, and still there are odd gaps.
To come up with a reasonable, "gap filler" category defined as "everything that doesn't fit somewhere else" may be out of reach. There would be too many questionable sites. They would be so diverse in nature and geography so as to result in quite a hodge-podge with the only commonality being something that is somewhow "historic" by various criteria. The list of exclusions alone would be a a jumble.
Just trying to be realistic here, that's all.