Wow, my english in the first post is even worse than the usual °L°
Yes, I arrived at the same conclusions talking to the players of the game I was writing the front for while the thought of the "scale" struck me.
To me, the trick for success is: understand what matters to the players and their characters; realize the best appropriate scale actually thinking in terms of trollbabe; and then create an Impending Doom that totally and permanently destroys the targets of their interest, but limiting the damage to the scale.
So for example, in the first session they emerged as stalwart defenders of the town of Larthan. Without hesitation, I started writing a danger aimed to annihilate Larthan. But then I stopped and I figured out that not only this would focus the game on that danger, without leaving space for anything else; but also that it would be too fast and too gratuitous. So I stepped back, posted here on the forums, reflected about it, talked to the players, and came up with three dangers that destroy important parts of Larthan, but not the town itself. Specifically: the noble house of larthan falls into oblivion because the last in the bloodline is killed; various packs of humanoid vermins raid the country around Larthan, driving back civilization; and the more fanatic branch of the main church in Larthan makes wizardry illegal and every practicioner of arcane magic within the walls is burned or exiled.