I found the parts about patching and frames with different blend modes very fascinating. I wonder if it would be possible to build a GUI DCC app that uses JpegXL as its project format. It seems that it could support layers, splines, symbols (transformed instances of layers), blend modes and animations without "baking" any of it to pixels
Yes, my hope is that jxl can become an interoperable format for layered images. It does not have all the functionality of image editor formats (PSD, XCF, etc), but it does have a very useful subset of that (named layers with basic blend modes). For interchange, it would be very suitable since it has state of the art compression (both lossy and lossless), does not force you to merge the layers before exporting, while the images can be viewed directly by any application that supports jxl — since the blending happens inside the decoder, the viewing application can be blissfully unaware that it even is a layered image, it will just get the merged image from the decoder if it doesn't ask for the individual layers.