I'd be interested in a format that can replace TIFF, without the files being quite so enormous. However, it seems like all the FLIF tools are assuming you're coming from PNG-land.
- multi-layer (overlays), with named layers
- high bit depth
- metadata (the JXL file format will support not just Exif/XMP but also JUMBF, which gives you all kinds of already-standardized metadata, e.g. for 360)
- arbitrary extra channels
All that with state-of-the-art compression, both lossy and lossless.
TIFF acknowledges EXIF and IPTC as first class data. PNG added EXIF data as an official extension a couple of years ago and I know ExifTool does support it but I'd want to check all applications in a workflow for import/edit/export support before trusting it.
TIFF supports multiple pages (images) per file and also multilayer images (ala photo-editing).
TIFF supports various colour spaces like CMYK and LAB. AFAIK, PNG only supports RGB/RGBA so for image or print professionals, that could be a non-starter.
So I get why PNG can't warm photographers' hearts yet. Witness still the most common workflows: RAW->TIFF & RAW->JPEG.