We have plans to implement a new file format that will provide this functionality. Basically, the new format will be a subset of the current XOSM format for a single image with optional embedded or offline pixel data (maybe XISM, for XML Image Serialization Module). We have no plans to provide any additional functionality through the FITS format.
Bear in mind that previews are not really useful without their mother images and all the associated processing histories and stored states. Now if you try to implement all of that functionality and data resources using FITS keywords and FITS extensions, well, it is indeed possible, but does not make any sense IMO. An XML-based format is much more efficient and flexible.