NWN made me tinkering and messing around if not temporarily even geeking out but to be honest I never managed to create something playable for some friends of mine which I was gamemastering.
Please don't make it like SCL, but please please please look at the parts it really got right!
My interpretation is that this is essentially the point.
The DOS:2 EDITOR ideally provides the depth I believe you've enjoyed NWN.
Whereas the GM mode is more entry-level tool that also requires less preparation. I'm hoping it will still be reasonably deep.
My point was that SCL Editor and GM Mode is almost the same thing. That means the Editor is pretty weak but the GM Mode while lacking a lot of functionality (e.g. GM has no possibility to inspect player characters and so on) is engine wise really strong because it is basically an Editor but in an live playing environment. See my example of "brooming away" characters with placeables, which i find really impressive!
I really don't know if the Divinity Engine has the possible capacity for "live building" (well terrain building seems pretty unpossible anyway but placing objects). I the Divinity Editor you have to switch to playtesting modus where you can steer characters but you loose the power to place stuff.
We will see once it's done. Just wanted to point out the possible horsepower of certain aspects of a GM Mode which includes a lot Editor functionality!