Some way of creating custom objects.
E.g. we have units and destructibles, but I would like the ability to create a new object type, like a missile or other helper.
Ability to add custom data to existing classes (like units), so that you could e.g. add a stat for Wisdom as a 4th hero attribute. These fields won't do anything by themselves, by you should have access to them in triggers/code. Support any data type - integer/real/string/etc
Make own categories/folders, instead of being forced to use the race/etc structure as it is in war3.
A way to import/export an object's raw data (text). So I could for example generate a whole bunch of waves for my TD using some external program (all in text) and import it to have the editor create all those objects.
Links to related objects (as in sc2) editor. E.g. if I'm making an ability and I want to edit the buff, I can click on an icon next to it in the buff list to take me there.
For simple fields (like just a number), the field should be editable without having to double click it. Click it once, it becomes an editable text field (like when you click a trigger second time you can rename it).
Ability to edit your "workspace". E.g. in 3D editing tools, you can add viewports for different angles and such. The war3 object editor has just the list and the fields on the right, but it would be cool to have a button to add e.g. another list.
Integration with the trigger editor. E.g. if you make a new ability, instead of an effect there could be a simple field saying "run trigger X".