One of the things that comes up when I'm working on a plot or setting for a game is source material.
There's a lot of inspiration out there within media in general not least of which is the Science Fiction and Fantasy section of your local bookshop or library. If you're going to re-use a published setting or indeed take inspiration from it there are some ground rules to consider.
Will you use the setting as written?
Will the protagonists and antagonists from that source material appear in your game?
These 2 questions have already surfaced in my plans for a Star Wars EotE game. Now it is Star Wars so most people relate to the characters in those films and in many respects the canon that the timeline represents. So do you stick to the canon?
You can play a Star Wars game that plays on the outskirts of the story from Acts IV, V and VI of Star Wars and indeed you can play a game where that story is rewritten based on the actions of the player characters. I've done both and something in between where the canon of the stories was practically irrelevant other than to set some setting specifics e.g. status of the Rebellion, how strong is the Empire's grip on the galaxy etc.
I suppose it comes down to what you want the game to be about rather than where the game is situated. Or to put it another way - Setting vs Context.
Having a Fantasy scenario set within The Shire from Hobbit/Lord Of The Rings is a great setting. The context of that scenario will define where in the timeline the scenario is occurring. Assumption there is that it will be part of the canon timeline. You could run a game set in The Shire using a "What If?" approach e.g. What if Bilbo didn't go on his unexpected journey? I'm not entirely sure what you'd do with The Shire in that context but at the same time I can imagine how it could provide a great setting when looking at it in that context. Choosing to retcon an existing setting to provide new options for play can result in some interesting stories.
Putting that context spin on a variety of established settings can make for a very different game.
Call Of Cthulhu is predominantly set during the 1920s and has a few context spins out there already. Spin it further and you could have it set during the Roman Empire with Cthulhu's agents working within the senate and having some sort of link with Neptune (God of the Sea).
Taking the context and skewing it can be fun but I suppose you have to go back to those original questions and then add - What do the other players want? - into the mix before deciding on a context.
After all being the Rebel Alliance strike team that helps to rescue Han Solo etc from a plan gone wrong sounds like fun too.