There was an interesting guest editorial in Fridays Times about forewarning.
Forget the usual worry about changing the future - go for it. The article was about going back and warning world leaders about the long term consequences of their policies, probably correct ones in their eyes at the time.
His changes included
The UKs immigration policies from 1950 onwards.
Avoiding the financial crash.
Less involvement in the Middle East.
What would yours be?