First off, originality doesn't really mean anything. Every idea has been thought of at some point, you just haven't seen it done yet. When I was younger and first read Eragon I thought it was so original, but I had never read Pern or Earthsea, so I had no idea it was just that stuff but Lord of the Rings-y. The same idea can be executed in so many different ways, you know what they say, the medium is the message, an idea in a movie or a book or a visual novel or an MMO or a mobile puzzle game will hit different because it's being consumed differently. Some of my favorite games can be described as "anime Saw" but they're so much more than that because of how you interact with the story and how you approach the themes. So I don't worry about "originality" anymore since, ultimately, it's the execution that really matters.
I have ADHD so most of my ideas just...happen. I am never at a loss for ideas, but I only have so much time/bandwidth to develop so many. They usually spawn as "what ifs": what if it only rained once a year? what if a unicorn lost her job because of the gig economy? what if each city had a spirit that personified it and they're forced to deal with growing globalization? what if emperor norton actually became an emperor? They might get triggered by something I'm doing (like if I'm walking around San Francisco, I might see the {Salesforce} Transbay Terminal and remember that there's a plaque in there honoring "emperor" Norton, man that guy was something, wouldn't it be funny if he actually became an emperor somehow?) or randomly. What I write is mostly speculative fiction, which fits perfectly into these concepts, but I also write a decent bit of fanfiction, too, and they also tend to be based on what ifs, but whatever is the funniest/stupidest/most dramatic (what if they were on a terrible reality show, what if I shipped the most toxic characters together etc).