I love this idea. Its implemented in some MMOs already and i think it adds alot to the game if these kinds of events happen sometimes that affects everyone.
Especially i like things like foreign armies invading the whole land and everyone has to fend them off, or an evil wizard takes control of supermonsters that ravage the land
How about huge forest fires that everyone in a forest region has to try to fend off, together? maybe someone will take this opportunity to attack a neighbor