If we replace those heuristics with a search that tries out which of the values is closest to the original, we should get better quality, especially at the lowest bitrates where smoothing is important.
It is unlikely that there will be any bitstream changes in JPEG XL. There is still a lot of potential for encoder improvements within the current bitstream, both for lossy and for lossless.
will those require a bitstream change too?
No bitstream changes. In JPEG XL we have a 3-bit control field for each 8x8 square to guide which pixels participate in smoothing.
The heuristics (here https://github.com/libjxl/libjxl/blob/main/lib/jxl/enc_ar_co...) for choosing that value are quite primitive and produces only two values: no smoothing and some smoothing (values 0 and 4).
If we replace those heuristics with a search that tries out which of the values is closest to the original, we should get better quality, especially at the lowest bitrates where smoothing is important.
It is unlikely that there will be any bitstream changes in JPEG XL. There is still a lot of potential for encoder improvements within the current bitstream, both for lossy and for lossless.