But what about the definition "God is the force which created the universe, and did so intentionally". How could you ever disprove that?
Or what about "God created the universe as if he did not". Unfalsifiable.
I'd dismiss the second one because God is undefined.
The first one is interesting. Intention demands some sort of consciousness or intelligence. And in fact, this is one of the most pervasive attributes of God, outside of abstract (non)definitions.
So by this definition, if we were to discover a conscious or intelligent force responsible for the creation of our universe, it would be God.
To say that God is what intentionally created our universe, we must either mean that God is a term we understand and it refers to the force which btw intentionally created our universe, or that God = whatever force intentionally created our universe.
Since God is given no other definition, we're left with the latter option.
Now, suppose we humans develop technologies -- and it's not unthinkable -- which spawn universes, ones which interface with ours merely tangentially. If intelligent beings evolved within one of these, we could be described as the force which intentionally created their universe. So it's not unthinkable.
If something of that sort were discovered to be true of our universe, though, I wouldn't conclude that we'd found God, but rather that God hypothesis was wrong. And although some (the Scientologists and the Raelians come to mind) might disagree, then what can we conclude? If that could be God, then what couldn't? And if anything could, as long as someone said it did, even if no one had ever believed it or called God that before, and no one could argue definitively against whatever it was, even if it were nothing, then the term is being used so loosely as to allow outright self-contradiction or fatal vagueness -- that is, used in a sense in which it has ceased to retain significance, to mean anything in particular.
So the latter interpretation doesn't work either.
We'd be forced to say, "God is one of the possible forces that might have intentionally created our universe."
So we'd need another qualification. I think this is where "supernatural" has to come in.