I made a plugin for JPEG XL.
It's a fairly new format, only standardized this year (2022). It has some very useful features though, not least of which is being able to losslessly transcode JPEG files with around 20% smaller output.
This initial beta release needs more extensive testing. It is rather slow and I am looking at ways to optimize it. ICC colour profiles are not supported either, but it's not clear exactly how the decoding works so I'd be interested to know if the colour looks right on files with those profiles.
Finally, HDR is not supported either. I'm not sure if it can be supported, haven't really looked.
Copy both the .dll and .lld into the Viewers folder and refresh the plugin list in Opus.