So the easy way to approach this is from the players point of view - playability. Select a SIMPLE theme idea. Don't bog down in the historical and technical. Keep it light and add to the theme as you need to. I write novels and term papers. Coming up with a theme is really hard because you want to do a lot that's going to read well. But with each project or paper, I make myself sit down and reduce the ideas into a single idea or theme. Throw out everything unnecessary and start with a solid foundation, then build a great "house" or er game in this case. Best advice I can give.
Themes How to or what not to do
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Step 1: Get Excited!
For Step 1, also go for the probable game's Joy Quotient first!
What theme actually excites you? What would you like to do, be or have? Don't start with an analytical mind. First, get those creative juices flowing is what I will recommend.
Then, go ahead and find what you need to know.