What can I modify?

There are a few tools available to create content:

Dungeon Importer / Creator

https://talespire.creadth.com/slab/import-dungeon the code is available at: https://github.com/creadth/Creadth.Talespire.DungeonGenerator

Random Forest Creator

http://d20armyknife.com/talespireforest/ code is available at: https://github.com/brcoding/TaleSpireHtmlSlabGeneration