JPEG XL's Modular mode is based on the best elements of FLIF/FUIF and lossless WebP. For lossless compression, it does improve quite significantly over FLIF, see e.g.
https://docs.google.com/spreadsheets/d/1ju4q1WkaXT7WoxZINmQp...
(FLIF is on average 7.2% larger than JXL, and for some categories like pixel art the gap is larger). It also decodes significantly faster, and it has fast encode options that make it more suitable as a PNG replacement in image authoring use cases where density is not the most important thing.
Progressive animation is a neat feature of FLIF but it is totally impractical in terms of decode speed and memory consumption. In general I think animation is better handled with video codecs (with multiple resolutions/bitrates to cover various devices/network conditions).