This seems a bit restrictive. In general there may be several other coordinate systems that can be used to describe positions within a data set, not just "pixel" and "world". Position within the telescope focal plane may be the most obvious, but there could be others. For instance, some sort of polar coordinate system with the pole at some object of interest could be useful in some situations. Or a version of pixel coordinates that describe offset from some specified origin. The possibilities are endless. The FITS-WCS papers recognise this, after a fashion, by allowing for "alternate" coordinate systems. But it is too restrictive again in my opinion. The most versatile way is to make your WCS object represent an arbitrary network of coordinate frames connected together by transformations, with methods to find frames that have specified properties, and to transform positions between any two specified frames.