Sometimes I have recurring drawings in my head—ideas I can picture clearly that I would love to turn into final pieces, as well as rough or abstract ideas that I know may not turn into anything. But because these ideas keep coming back, I've found that I often simply need to capture them and put them down on paper (or on my iPad) to free up space to think about other projects.
This is an important part of my creative process as an artist. Not sketching them out will mean I either lose them (less likely) or they will continue to occupy my brain and prevent me from working on the projects I need to prioritize (more likely).
Here's a recent example of a recent example, with a little Ennio Morricone soundtrack:
This cowboy/gaucho was something I found myself coming back to, but I didn't have an outlet for it in any projects I was currently working on. I also didn't want to lose the idea, either. So, I sketched it out, quickly, loosely, and purposefully.
Sketching freely like this is freeing and a good thing to do creatively. Ideas flow and the practice helps improve your drawing skills.
If you're like me, your inner critic can be harsh. Way too harsh. But listening to that and not sketching ideas you're unsure will work only hinders your ability to create. So, my advice is to just draw. Don't worry if it looks right, just draw.
Doing sketches that aren't necessarily for a given project or goal is also a chance to try different styles or try new tools (or new digital brushes, if you're working digitally). You might discover something new you like.
So, my advice if you're stuck in a rut or want to generate new ideas? Put whatever is on your mind down on paper. Start there. That output will help you free up space in your head (and develop new ideas).