Here is the relevant bit of that post in relation to your comment:
The thing that might have the biggest visual impact now would be to redo lighting and the way materials are rendered but that isn’t really feasible unless we move on to GD2 and leave all this current art behind. If we go on to GD2 and no longer have to support older DX version or existing art, then we’d be free to make bigger updates to the engine.
There might be other posts where he touches on the same topic but that’s the first one I was able to find.