Although you could take an existing ostree commit and add changes and repackage it into an ISO that’s not the easiest way to go for creating custom Fedora CoreOS images (but it is how changes can be applied via rpm-ostree overlays on a deployed system). You should probably start with the CoreOS assembler README and tweak the existing Fedora CoreOS config.
Building an image not based on Fedora will be more work, especially if you’re choosing a non RPM distro.