My litmus test for whether an idea has merit has always been to ask myself “Can I make this happen by myself? If not, can I make it happen with a 1-3 people I know and can work with?” If the answer is “No”, then it’s probably time to toss the idea out or rework it until the answer is “Yes”. As I’ve gained experience and grown as a developer, the scope of ideas I can answer “Yes” to has steadily grown. I’ve also come to realize that a lot of my early ideas were stupid - they might have had some interesting aspects to them, but they were either too elaborate or just impossible to implement. Keep in mind that “make it happen” could mean “build a prototype to demonstrate the gameplay or principle at work”, not necessarily an entire finished product. Remember, ideas are only good if they can be made a reality.
Ideas are not special snowflakes that need to be nurtured and protected from the harsh realities of the world. Ideas are extremely common. You should always be ready to change or discard your ideas and come up with new ones - nothing is sacred. Ideas exist as potential solutions to a problem. If the idea cannot actually solve the problem, then you must discard them and move on to one that can or revise and fix your failed idea to make it work. To do this, you need to be able to evaluate your idea for feasibility. What are its possible failure points? What assumptions does the idea make? What happens if one or more of those assumptions is wrong?
Remember, pitching an idea is more than just getting someone to buy into the idea itself. Whenever you pitch an idea, what you are really pitching is your ability to make that idea a reality. Nobody is going to pay for someone else’s idea only to do all of the work themselves, not when they are fully capable of making their own ideas a reality. Which goes right back to my litmus test - if I can’t do it myself (or within a small team of people I actually know and can work with), then it’s not a feasible idea and I need to come up with something else that I can do. If you go into game development, you’ll often be asked to come up with ideas to solve problems. In those situations, coming up with an idea that doesn’t involve you solving the problem won’t fly. Any idea that actually gets used will almost certainly require you to make it happen. As your skills improve and you gain experience, the scope of ideas you can say “Yes” to, as well as your ability to determine an idea’s feasibility, will also increase.