Miracle Days and when everything changes

101 Different Miracle Days. It hasn't reached 101 yet, but we'll see.

SF premises that would be wonderful in some ways but could totally mangle the world.

Of course, you have to consider the effects on the players' characters. Does the miracle tie in to the special powers of a PC?

For example, how would it alter play as well as the world in general if everyone affected suddenly developed telepathy? Including the PCs, so now you have to deal with near-total knowledge on the part of the PCs? Or excluding them?

This is definitely something to think about before starting a plot along these lines. As well as the world, changes to humanity as a whole would affect the PCs, changing them from what the players originally envisioned. Reassuring them that this is temporary will help in most cases, but not all. Imagine something like The Midwich Cuckoos - "Immaculate conception. For every woman able to bear children. All at once." Then consider the players of female PCs. That could have effects far beyond not being able to die for a few months. The more deeply it affects the characters, the more problematic it may be.

Army Of Ghosts doesn't touch anyone physically until the Cybermen arrive, but we see how it affects society, in some cases deeply affecting people. That one step away might make it less troubling, although it could still hit raw nerves.

When something changes the setting profoundly, and doubly when something alters the PCs themselves, you have to make sure the players will want to take this on board.