So here is an abstract way to think about and maybe keep track of these happenings in 5 stages:
- First-hand Tale
- Personal Experience
- Witness a Mass Event
- Unavoidable Consequences / Civilization Interrupted
Rumors are clear. Stage 2 has pcs encounter a person who has witnessed the happening themselves and can give an eye-witness account. A stage 3 things are becoming so common that the pcs are the ones to witness something. Stage for the pcs witness something again, but now it is bigger and more dire. By stage 5 even if the players have avoided interacting with this event for session after session they can't now, merchant, towns, trade, lodgings, even camping safely in the woods will be affected.
When do stages escalate? I say that's up to you as DM. The simplest of course would be to kick it up a notch each session of play. But you could make some happenings build slowly to a boil over the course of a year of real gameplay too.
I don't think every happening would have to end at stage 5; if every happening has unavoidable consequences, eventually the players will realize this and say “Ah, screw it, we've got to go deal with this war.” Happenings could resolve themselves, or be resolved by other parties. Or in an evil campaign a happening may never escalate past a certain level unless players push it gleefully along.
Now, I don't have any experience running a sandbox, so if this is all old hat forgive me. Here are some ideas off the top of my head of things that might happen in the background:
Possible Dire Happenings
Seems pretty straightforward even if players never get involved in actual fighting they'll experience raids, looting, and the wreckage of places armies have been..
Check out the Vaults of Nagoh's memestorms and ideocults for great ideas regarding this category. I think the idea of the Dance of Death is really archetypal of the dark ages too.
While many disasters are eye-blink fast, I think a few would work well with escalation: the earth has been rumbling for weeks, faults have been opening up underneath people, before the big quake hits. Or, the mountain has been smoking, someone saw some lava streaming down the opposite face, before the huge eruption. Or maybe, for some reason, it . . . just . . . won't . . . stop . . . raining.
I sort of like the idea of deciding ahead of time that the player characters will be immune to whatever pestilence is spreading, and having them deal with the envious npcs and any guilty feelings they might have that they are unscathed.
My first thought was dragon, but this could work for a Giant or a Vampire and his followers, anything that can progressively become more of a problem.
Not-So-Dire World Events
Not everything that happens in the world is potentially world-ending, maybe there is just a fad of people wearing Red-checked hose. So, what might happen that would be interesting/important enough to notice but not so dire?
- Memestorms/Ideocults could fit here too
- Religion on the Rise (the cult of the Red-Checked Hose)
- Cultures Mixing (An Influx of Dwarves)
- Guild Strife
- Newly Chartered City
- Trade Shortage
- Noble Intrigues
What background happenings have you had going on in your sandbox?