I've occasionally pondered, how feasible would it be to write a APFS implementation just from the specs[0] alone. Is it harder or easier to create the implementation when you have a provided layout and mechanism how it works. Would it be easy to keep compatibility, and would it be a dead-end design for extensions that you'd like?

[0] https://developer.apple.com/support/downloads/Apple-File-Sys...

Can tell you first hand that while the specs are useful, they do not cover APFS as it is deployed by Apple today.