Implement multistage bootstrapping
mkarchiso relies on multiple commands on the system that runs mkarchiso. E.g mkfs.fat, mksquashfs, mkfs.ext4, xorriso, coreutils, etc.
These software can greatly affect reproducibility. To ensure some consistency, we should record the used package versions on the system that runs mkarchiso. To do that we'd need to limit the scope of package versions to record otherwise it could become a privacy issue for people running mkarchiso. Doing so, though, would be error prone.
A solution suggested by @klausenbusk is multistage bootstrapping. I.e. create an Arch container with the required packages and run mkarchiso from there. This would be fully reproducible.
If implemented well, this could even allow running mkarchiso from other Linux distros.