Would very definitely like to see a more robust art import process that includes the ability to bring in custom rigged meshes and animations made in outside 3D packages. This is my own primary interest in the editor.

Friendliness to small mods seems really important (this includes a lot of things that have already been mentioned vis-a-vis mod limits, ease of use, not having mods tied into the main campaign, etc.). The DOS editor in its current state seems to say "who's going to bother installing a mod that adds a small new location, or a few new items? Go big or go home!"

More thorough documentation. This sort of goes beyond just tutorials -- I can imagine there isn't going to be a tutorial on every little thing the editor can do. But a thoroughly documented editor might be achievable, and would go a very long way to making the editor viably usable by modders. This approach would make tutorials more suitable for just teaching the basics of using the editor, with documentation handling the more advanced features.