When it comes to creating something new, I'm all about taking the tiniest of steps. A simple mark on paper is all it takes to start. Keeping project ideas just in my head is a bad idea, but I know I have my reasons why I might leave one there to marinate: A.) I think the idea is fully baked and perfect, but really I'm just afraid to test drive it and watch it fail. Or B.) the idea really has no teeth, really, and I kind of know it, but I don't want to face a test drive and find out for sure.
So, yeah, leaving said idea comfortably in my head is a great way to never face up to whether or not the idea might actually work. It's a lot easier to just dream about it and wait for the big lightning bolt to strike and make it all right from the start.
The creative process doesn't work that way, unfortunately.
Use your pencils. Make some marks. Start small.