I know what you mean. I so know what you mean.
Ideas, (for me at least) come in certain 'varieties' that will help me initially slog through them. For example, some ideas sound better to me written down. As in a book or short story or something. Others sound better as game-plot mechanics, while some...just seem best as movies. @-@
When I come across a 'game mechanic idea', my next step is to begin jotting down ideas for it. (Like you.) This is the fastest way to start forming what you're thinking in front of you. However, I do tend to stick with this idea for at least a few days and horde more concepts for it. If I'm still interested, then I move onto the next factor. (and the biggest, if you ask me).
Resources.
It's great if I want to make a pre-historic world crash into Candy Land and have the rainbow-colored protagonist ride a harpie-dog-pig-door around a world of nothing but jelly beans. But can I really scrounge up the graphics, faces, and music to bring such a haphazard idea to life in a way that's eloquent? Or even interesting?
On a normal note, even if I wanted to make a Western, resources for settings like that are also scarce. And of course, modern-futuristic tiles are only just now really making an impact on the collectibles out there.
So, I tend to bend my ideas toward what I see most often. What can be easily edited or mixed and matched to come as close to the vision as possible, while still offering diversity. One can do a lot with minimal tiles and graphics, as seen with all of the RTP made games around here, but having so little isn't always encouraging for a developer, so it's person to person at this point. (And don't be afraid to trim down the ideas so that you can make the most of limited tiles and such. A game can be short as well as fun!)
Basically, once I think I have the tiles and ideas together, I begin work on everything else. It's perfectly normal to have many blank projects that don't go much past a single boss fight. (If it even makes it there.) But I have found that sooner or later, one or two tend to stick out above the rest.
You'll begin to realize just what making a game means; and you'll be able to spot which ideas you can actually stand the test of time with, and which ones are just lead balloons.
This may not happen any time soon. But if you're persistent and dedicated, it's bound to happen eventually. And then, you'll have your wings to fly with~