With the addition of update 1.6, we saw the introduction of the Song Selector.
While I had not done extensive testing since this update, since I had seen no mention of it in the patch notes then I am assuming for custom music mods we still need to replace song levels.
With this in mind, my question would be on the possibility of having mod support to add levels to the song selector rather than replacing.
A form of this could be such as Song selector has N number of slots with the first few being taken up by base game music. When adding songs, in the JSON rather than specifying level name, you would instead specify selector slot number between 1 and N. Additionally there could be another option whether it would be a Boss song or standard level song to determine which selector it would reside in.
Again I had not tested since the launch of the update though this request was made based off the information in the 1.6 patch notes as well as the Modding documentation still indicating the requirement of specifying level names.