That's right, this handles transformations with different properties but the output coordinate system is the same. This was mentioned as a requirement because FITS and `astropy.wcs` do not support it. Of course `AST` does. The [SelectorModel](https://github.com/nden/astropy-api/blob/21b0d36eb0677de254a5554a5cf9b28fdf7f6540/generalized_wcs/selector.md) will handle the details of this, but otherwise the WCS api will be the same. Do you see a problem with this approach?