I'd like to invite you into my brain for a moment. Don't mind the clutter. People ask me where I get my ideas from for my books and I never can give a very satisfactory answer, but here's an illustration.
I was lying in bed not wanting to get up and my mind wanders as it does. I thought about the idea of how some people would like to go back to high school or college with what they know now and do it differently; I thought about how I would not really care for that.
Then I thought of how time travel is considered in stories, like a short story I read once where someone went back in time and killed Hitler then came back and nothing had changed. In Marvel Comics, John Byrne (I think it was) came up with the idea that you can't ever change the present by going back in time, because the instant you do so, you've created a splinter, alternate reality. Any changes you make in that reality like showing up are in that universe, not the one you came from.
Then I thought about a sort of elastic, pocket version of this. What if time travel worked like this:
You can go back in time, but you create a pocket universe when you do so. Everything you do there is contained in that pocket, and when you leave, it resets back the way it was.
Then I speculated that it isn't really you that goes back in time, its a copy of you, a duplicate body with your consciousness. So if you die, you just get booted back to the present. Even if you die of old age.
Then I thought about what that would mean for a culture and how people would use it. Folks would use it for entertainment: go back in time to the Civil War and fight in it, go back to the Roman days, go back to Medieval France and fight in a tournament. If you die, oh well.
Some would go back and live entire lives in the pocket, aging and dying, then coming back with all the memories and experiences. Travel through time, woo and marry Helen of Troy, prevent the war and have little perfect babies. Die of old age, and come back.
It would be used by some for research: what was it really like during the Napoleonic wars? Some would use it for just dark desires in their soul, to experience being a conqueror or murderer. Some would just use it for weird sexual escapades.
Some would use it to try out alternate scenarios: what if this happened? What if someone shot Hitler just after he got into power? What if someone killed Lenin and the Russian Revolution never happened? You could play out all those alternate history scenarios.
But this technology would almost certainly be available only to the most extremely wealthy or fortunate, not just anyone because it probably would be very rare, difficult to use, and expensive - if not heavily regulated.
Then I thought about how people might use it to try something else: go back in time, say, ten minutes, then live out various alternate scenarios to see what works best. Should I marry her? What if I invest in this? How about if I leave my job and take that other one? Will this unbelievably dangerous experiment succeed? Lets try out 600 different slightly changed experiments to see what gives the best results.
But then I thought about what happens after they try this. Say you want to create an advertising campaign, and you use time travel to test market it, and get the most successful one. Great idea, right? Except... is your competitor doing the same thing? Your pocket universe is isolated from the rest, so you don't find out what happens when other people are manipulating time.
So someone could try out the ultimate scenario and get it exactly right in a pocket universe, come back, and find out it all goes horribly wrong.
And there was the plot for a science fiction story. Someone goes into a pocket universe to test out their theory or scheme, gets the right one in a sort of Groundhog Day type of sequence of trial and error, and then triumphantly unveils it on the world... to see it all go horribly wrong. A decent short story.
And that's where it all comes from. I just think about some ideas, follow them through to the next possible event, ask questions about what would happen next and why, and away we go. Its not always successful, but its what I do when I am going to sleep or waking up or have time to kill. Thousands of stories I've played out in my head, mostly nonsense and silly stuff but sometimes a good idea pops up and I write it down. You probably do the same thing, but writers keep track and follow the idea through.
Like the idea of heroic version of Jason from Friday the 13th. If you can be an unkillable terrifying monster why not an unkillable good guy? Or the fantasy sea novel; the story of a water wizard (who you've met if you have read Old Habits) in a Horatio Hornblower type series. Or the series of short stories about a war horse in the pattern of Call of the Wild. And so on.
That's where I get my ideas. My brain takes all the weird input and interesting stuff I experience, learn about, and see and sorts them out into little tales. And some of them end up on paper.