The problem is that there is a big difference between an idea that is within your actual ability to see it through verses an idea way outside any abilities you could even hope to develop in a reasonable manner, and it isn't always clear which is which as we tend to way overestimate or underestimate what we can do and the time we can do it in. I explore all ideas, regardless of whether or not I can actually do anything with them or not, because it can lead to some really cool breakthroughs but I've had to get better about what I actually choose to undertake as a project. Even when something is within my abilities it can still be beyond the time I have free to work on it, beyond my emotional stability, beyond my attention span, etc.
That all said, once you have a fair idea of what you can actually afford (in time, energy, or finances) ideas are pretty easy to come by, it is worthwhile ideas that are tricky. To get to a worthwhile idea you're going to have to explore a lot of bad ideas and squish ideas together and tear pieces of ideas apart and Frankenstein that shit over and over. Eventually you'll find yourself coming back to the same sort of direction over and over and that is when you are potentially on the trail of something good.
There's nothing wrong with using the ideas of others as ingredients, nearly everything we do is some kind of combination of things done before. How you choose to go about combining and what you choose to go about combining is where your originality and personality come into play, it's what will make the project feel like it is uniquely coming from your perspective and tastes.
If all else fails, you can apply the approach of one of those "fusion memes" and take three established ideas and see the ways you could combine them:
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