I send $25 to a P.O Box in Poughkeepsie and they send me a six pack of ideas.
Sorry. Had to get the Ellison out of my system.
I admittedly come at this from a slightly different perspective than some of the other folks on here, as I started with novels, not with games, so grain of salt on my advice. My problem also doesn’t tend to be lack of ideas as much as lack of follow through.
A big thing to try is pinning down why you feel like you don’t have that thing in an idea that lets your write it and get hooked. Are you getting bogged down in “originality?” There are only three stories in the western world: man vs man, man vs nature, and man vs self. Everything else is a riff on those three. Or there’s 36 stories. Or there’s 7. Whatever tack you take, originality isn’t the end all, be all, and is frankly a bit overrated. One of the most successful franchises of the past decade is literally fan fic with the serial numbers filed off.
However, I do have a method I like, stealing shamelessly from Joshua Palmatier, the Editor in Chief of Zombies Need Brains. I can’t track down which random blog post it was in (When they put up their anthology calls each year, they announce each one on a different blog, with a new piece of advice on writing for the anthology on each of them.), but the method he recommends for coming up with something that doesn’t feel either too derivative or too out there is as follows:
Write down 20 ideas, all on some theme (If you’re inspired by something in a WIP, use that a starting point, or make it 20 ideas with one genre, or 20 ideas that share one game mechanic. Whatever works.). No more, no less. Then, axe the first and the last five of them, unless one really screams at you that it’s right. Typically, those first five ideas are going to be the most derivative or obvious interpretations, and the last five are probably going to be a little too disconnected, esoteric, or out there. Somewhere in the middle ten, you’re likely to find that sweet spot.
I would also recommend giving this talk a listen. It’s from Mark Rosewater, who is creative lead on Magic: The Gathering, and has a lot of great advice for creativity and creation. The GDC archive in general is a great resource, but I’m very partial to this talk. Gets me fired up.