So you want to make a new rpm-ostree desktop variant in Fedora to join Silverblue, Kinoite and Sericea? Here are the steps that you should follow.
First, you should figure out what packages should be installed in your image. To do that, you can start from the base container image (https://quay.io/repository/fedora-ostree-desktops/base) and add packages from the comps group corresponding to your desktop in Fedora : https://pagure.io/fedora-comps
At this point, you should consider which apps are going to be shipped by default, included in the image, and which ones are going to be installed by default as Flatpaks.
Once you've got something working, reproduce that in https://pagure.io/workstation-ostree-config using the comps group and the filter and sync script. Look at other variants for example.