Just a quick suggestion. I noticed the new sortOrder setting in the level jsons, and I thought it might be a good idea if the default sortOrder values were changed a bit just to make it easier on modders who want to insert a lot more stuff in between existing floors. My idea is to have the default order set to:
Dungeon: 10
Cave: 20
Sewer: 30
Undead: 40

Yes, this would break existing floor mods' order, but it might be better to do this at this early stage rather than wait until there are tons of floor mods released. I know we can override this already as it is, but that could mean mod incompatibilities if multiple mods start messing with those same files. Having more room in between the vanilla sort orders could help lessen the problem if you start adding lots of floors later on via mods.