@wizz
A war.
A revolution or rebellion.
The rise or fall of an empire.
Diplomatic relations and allies.
The arrival of magic.
The conflicts between conspiracies.
A change in technology, or economics sources.
The economic and social changes of a society.
Being conquered/invaded.
Building up a military force, a township, a new state.
Shifting alliances of a senate, Merchant Houses.
Probably more than one of those at a time.
It's a great deal of work, but I think the main barrier is that most players don't want an environment that can change significantly, especially if some of that will be not completely under their control (eg maybe they could mitigate a situation, but not control it completely. Maybe something bad happens to most or all, or a rival benefits more from something.) It wouldn't have to be fast change, but results would have to have weight.