Get your campaign map up to this point and prepare to wreck it. Do the following for each element or hex or grid or whatever you have pinned down. Alternatively, just make it up as you go.
Pull it in or push it out. Coastal towns will probably turn into villages, create outrageous canal systems, or just disappear. If it's gone then push it right off your map. It's out there but something weird happened.
If it shrivelled up then everyone on the river is worse off. Rich towns aren't any more. The bottom of the rung remain largely the same.
Swelled rivers will have new lakes and be more of a nuisance to traverse. Possibly new branches have split off
Dried rivers will have gone entirely. Possible reasons include sinkholes, erosion, ice, act of gods etc.
If there's water nearby, just smash them down if they shrink. Otherwise turn them into hills.
If they grow add some mountains along the general line of them.
If they're gone then consider volcanoes or more outlandish excuses. They could now be craters, or lakes.
Use this roll to get an idea of how much time has passed. If you roll lots of weird stuff for mountains then it's likely a lot of time has flown by since the players got themselves killed.
Growing and shrinking is obvious. Come up with a sentence to explain what happened in the missing time.
If a town is gone entirely 50% chance it's a creepy ruin inhabited by something awful. Or just bandits.
|Things got weird while you were dead|
Now, add some new towns in places that are begging for it. If in doubt, add 1d6 worth.
Now roll a some random events for the map area
- A bunch of foreigners conquered it and ... 1. left it in a right state. Roll for all the towns. 1-3 they were levelled 4-6 stick with the old roll 2.became natives. Add some exotic flavour 3. improved the general state of things. Aqueducts, roads, less wandering killers for hire etc. 4. are now the ruling caste
- Meteor. Split your map into a 6x6 grid, then roll 2d6 to figure out where it smacked down. If anything breakable is in the rolled grid, it hit that.
- New overlord moved in. 50% he's in charge now/in an advanced state of working on it
- It got colder. Add some snow
- It got hotter. Dry up those lush fields
- A new religion swept through. Roll a D6 for each old religion: 1: no one remembers it 2: its a secret cult that you wouldn't recognise 3-4: it's been absorbed by the new religion 5: its an unpopular minority 6: it coexists (50% happy/hostile)
- A significant monster has moved in. A dragon-esque thing. Everyone on the map will know about it and have opinions on it.
- There is a mysterious (or not so mysterious) plague afflicting the area nowadays.
- Technology has moved... 1-3 backwards. Think, medieval to bronze age 4-6 forwards. Medieval to renaissance. Don't think too literally, take a minute to imagine what "technology" means in your situation. Discovering lost knowledge? Or maybe losing that knowledge. It does not necessarily mean things are better.
- Anarchy! The largest governmental body is local. If it was that before, then pull it down to familial.
- Pick an omen or prophecy you've mentioned in your games. That came true.
- A totally new motif has taken over. If your people were scrapping in the dirt, now they're rich and fat. If they were a magocrcy now they're a totalitarian republic. If they were Byzantium now they're Turkey. Pick a fundamental change and roll with it.
Once all that's done, look at the map and tell yourself stories about what happened. Look at all the old points of interest and think how the changing world messed with them. Did a secret cult become that new religion that everyone loves now? Did that necromancer become the overlord?
Really mess with it. The players will visit familiar places and see how they've changed. They'll have insight into history that you haven't had to feed them. They know'cos they were there.