I didn’t watch The Man in the High Castle but I heard very good things about it. People I know who did watch it were gripped by the drama showing an alternative world to ours where Germany won World War Two and Hitler was a very old man. I don’t want to say more than that - no spoilers here! - but you can see how you could create your own timeline based on the opposite of what happened for real and create a whole new world and set of stories.
Another good starting point would be to take character traits you admire or loathe and get your characters, in a setting or time of your choice, to have the exact opposite!
For example, if you loved medieval life, what would the consequences be for, say, a knight who is a coward? (Okay, you could argue that knight would not live long and how did he get to be a knight in the first place with an attitude like that anyway? I suspect there would be stories to be had answering that question! Also, so much depends on perspective here. The knight might not be a coward at all - it is how he is seen by others who might have their own agendas here. So you can see there are story possibilities there as well).
So compare and contrast and have fun!