I don't know what you should write about, but once you find something, write as much backstory as you can. I have been writing backstory for one idea of mine for more than two years. I also have three other ideas in a similar state of progress.

As for a Metroid fiction, just play the games again, and think about what Samus would actually be thinking. What kind of thoughts would she be having as she battles, and how does she solve puzzles? Then try to find a plot in those ideas. That's what I do when I write about anything that I did not come up with.

I hope that helps.

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