We've started planning and implementation for mozjpeg 3.0. Most features
will be initially developed on branches, like the DC trellis quantization
branch:
https://github.com/mozilla/mozjpeg/tree/dctrellis
You'll probably see similar branches for new features soon.
You can see what we're planning for mozjpeg 3.0 by viewing our github
issues tagged with v3.0.