Obviously, the new editor needs to be simply and intuitively designed. We shouldn't have to look up a video tutorial to build a wall, for example. But I'm sure Larian is well aware of that.

The only big thing I can say for CERTAIN that the editor needs, specifically, is a robust dialog editor. Please, please, please look to the dialog editor in Neverwinter Nights' Toolset for an example of how to do it.