Post it on BitcoinStarter, and keep one critical aspect of your game idea undisclosed or undetailed?

Ideas are less steal-worthy than you might imagine.  As you'll no doubt have noticed, a great idea also requires a lot of effort to make it happen, and most people who'd steal are looking for something effortless.  That's kind of the point of stealing.

Also, be aware that if a good idea occurs to you and you do nothing with it, someone else will eventually get a similar idea and implement it anyway.  I got the idea for BitcoinStarter about a decade ago, but couldn't raise the money to fund development.  Someone else did, and it looks as though it will become successful!