I think it would be fun as well to have different rules per challenge, and a wider interpretation of “raw”. I can imagine that it would be interesting what people get out of an old, scanned photograph. And I think it could be interesting do have different “rules” depending on the picture. E.g., for the scanned old photograph, allowing GIMP/Krita/etc. would make sense. Another challenge idea, a bit more design oriented but still something in a photographer’s responsibility, would be something like “make a triptych from these three pictures (e.g. with 3 totally different pictures color- and contents-wise), allowing inkscape and scribus. Or make a photobook cover with the given image and a given title. Or allowing only command line tools for a certain challenge. And so on.
Disclaimer: I did not know about “PlayRaw” until this thread, and I tried to get information what it is about from the old RT forum. Unfortunately, the links there do not work any more and I have only a vague knowledge about “PlayRaw”. It could be that my writing above does not make sense at all …