For this editor to be the best that it can be you need to include:

Assets

1. Publicly shared assets

Have an online database of assets, user ratings, and an easy way to include them in the projects. (including models, animations, music and sound effects)

2. In engine asset modifications

Include a way to change the assets and doodads, and to paint with them. The painting tool should have a controllable randomizer for size and density, and a controllable density (as a minimum).

3. Animation and rigging

Perhaps cloth and hair rendering and materials is a bit much to ask for, but we at least need a way to attach animations to models. An animation editor for people to share animations and a rigging tool to attach the assets to these animations.

Programming Tools

4. Triggers

Easy and clear triggers that can be set up in the world map with a large pool of conditions to chose from. Even conversations with NPCs should be based on a trigger system.

Example: (asset 076: on player interaction, do subroutine "conversation 013") or (on player interaction, condition inventory "item 023" and "skill 011", do subroutine "conversation 013")

5. Easy to use subroutines

After a trigger condition has been met and a specific subroutine has been called, we need a clear and easy way to manage, edit, view, and understand the subroutine.

We need to be able to graphically create a flow chart with operators between events.
Example: event 1 conversation.
conversation option 1, conversation option 2, conversation option 3, operator test inventory item 011: conversation option 4

6. Debugger

An easy way to check our subroutines and triggers.