There are several image formats out there which I cannot find a Paint.net I/O plugin for yet. I wish there were:
AVIF, which is AV1-based HEIF (built upon ISOBMMF), unlike the default HEVC/H.265 in HEIC that is supported since 4.2 (if Windows supports it). There is a free download for an AV1 video extension in the MS Store, which might be enough already in combination with the HEIF download. (Edit: 4.2.2+)
BPG, HEVC i-frames in a different container
JPEG XL (JXL) and JPEG XT (JXT), incremental, partially backwards-compatible enhancements of classic JPEG
FLIF, a free for