After communicating with serainox420, I have brainstormed some ideas for an API.
Goals: To allow users to add their own special features to the mod instead of relying on me adding your suggestions
Things you might be able to add:

• Item factories
• Get commands
• Preset items (like the colorcodes book)
• Container interfaces (like opening armor stands with Ctrl+Space)
• Menu generators (like opening parsable strings in the editor)
• Config options
• Inventory tabs (like the client chest tab)
• NBT formatters (like in the value field in the editor)
• ItemLostCallback

Are there any other features you might be interested in?

Details:

• You would add everything via a mod that depends on the NBT Editor mod
• I won't use source code from these addons, but I might make my own implementation and add it to the official mod (I have lots of ideas written down that I would like to add, and people could easily end up making an addon for it before I do)

Please comment your thoughts on this concept. :D

This has been added in the latest version (v1.10.0). Use NBTEditorAddon as an nbteditor entrypoint. NBTEditorAPI provides the main methods. Use the multiversion package if you want to support all the Minecraft versions supported by the base mod.