Challenge ratings are contentious, but I’ve always held that it’s better for the GM to be informed than surprised. Problems arise when the GM is over-reliant (and therefore uninformed again) or the players develop a sense of entitlement. Let’s presume we can avoid both of those.
There’s a general challenge metric discussed on Mordite Monday: the Monstrous Obstacle. It basically ranks monsters by their average expected successes in a given conflict.
The trouble I see with using Danger level from the expedition is that it only measures danger to the expedition. There are many instances where something has a higher or lower danger than would be reflected by its menace in the adventure phase. The dragons are a great example. They have a high danger, sure, but they’re basically unbeatable in the adventure phase.
But taking your idea at face value, I’d say a territory’s average Danger level somehow correlates to an appropriate Drive Off “Monstacle” from the Master’s Manual articles.
So are you suggesting something to the effect of using an average of Danger Levels in a territory to set a monstacle? It’s interesting in an ecological sense.