One large area that syzkaller handles poorly is filesystem fuzzing: both mounting custom images and then executing random operations on these images.
What's exactly the best way of doing this is an open question.
We could either describe format of images in syzkaller descriptions and then let fuzzer generate/mutate them. The questions is: if generation of correct images is too complex for a full-kernel fuzzer? We will probably also need some extensions to the language to handle offsets/aligns/large zero paddings/etc.
Or we could maybe write some custom procedural logic to generate/mutate images. May be simpler, but worse long-term (more custom procedural code).
Or maybe something else.

FTR here is a good description on ext4 format:
https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout
Are there similar docs for other formats? Would be useful.

An interesting prior art is: Fuzzing File Systems via Two-Dimensional Input Space Exploration.

Some other prior art (though it's mostly "AFL on precollected corpus of images"):
https://lwn.net/Articles/685182/
https://events.static.linuxfound.org/sites/events/files/slides/AFL%20filesystem%20fuzzing%2C%20Vault%202016_0.pdf
https://www.blackhat.com/docs/eu-16/materials/eu-16-Jurczyk-Effective-File-Format-Fuzzing-Thoughts-Techniques-And-Results.pdf
https://www.blackhat.com/docs/eu-16/materials/eu-16-Jurczyk-Effective-File-Format-Fuzzing-Thoughts-Techniques-And-Results.pdf
https://lwn.net/Articles/637151/