tl;dr: We don’t take them. Thanks, but no. Best of luck making your game.
Why is a little more complicated. Even when I was still actively looking at our own development projects, our problem was never really a lack of ideas, it was a lack of time for the ideas we had. It’s naive to think that just the idea is enough, that the idea just starts good, or that it is already so good that it ‘just needs made.’ Ideas are cheap. We have them all the time. Working in games, playing games, watching what others are doing, ideas come all the time. Some better than others, some already made by other teams. But realising that idea takes time, and by extension, money. A lot of it. Even the best ideas are worth very little until they are implemented. Not even fully implemented, just getting the idea to a state where it can be pitched to a financier for funding takes a non-trivial amount of effort. It’s important to realise that whenever you have an idea you are trying to progress, what you are doing is investing in that idea. How much you’re investing depends on how valuable your time is (both to you and others), but you should always be aware that it is costing you to make this idea a reality, and that if you want it to succeed, it needs to realistically be able to pay you back more than you put into it.
Amateur developers often misjudge that equation. Their time spent on the idea is cheap, because it is enjoyable time, time they might want to spend anyway. Their assessment of the merits of their idea is often inflated because they are passionate about it. I think it helps to consider: “what if I had fifty ideas?” Then, time spent on one idea is time you can’t spend on one of the others. It forces you to think critically about the value of your time and on which idea really merits the investment you put into it.
That awareness that you are investing should also temper your desire to get other people involved. Because you are, effectively, asking them to invest into your idea. It is no different from going to friends or family and asking for a loan to start a business. There are some fundamental questions to ask, even before you get to the idea itself. “Yes, we want to help you. But what do we get out of it? What are you putting into this? What are we putting into this? Who gets what when it does well? What if we disagree on how this business should be run?” For these reasons it’s often easier to go it alone, at least to start with, or to only throw in your lot with people you know and trust. Because you don’t have to persuade those others of the merits of your idea. If you put the effort in, if you progress the idea to something that by itself can demonstrate that your idea has legs, then you start the relationship with your collaborators on a much better footing. “This is what I’ve done, this is what I think it’s worth, I could use a hand getting it finished, and if you agree with me on its merits we can come to some agreement on how to work together.”
This may be the jaded viewpoint of a professional developer, someone for whom new ideas and projects have to be traded off against lost income from other work, but I think all developers, amateur or not, should be thinking about their work not just in terms of potential but of cost. Because while this idea you have might be good, the next one might be great, and if you sink all of the investment you had to spare in the first idea and it doesn’t pay off, we’ll never get to see the great idea come to light. The ideas can only be realised when the development is sustainable, and to be sustainable requires the ideas to, on average, pay for themselves. Some amount of up-front investment is fine, but at some point one of those ideas needs to pay enough back to cover all of the ideas which didn’t.