Do you want to create a scene that causes viewers to suspend disbelief as they view a subject placed inside a scene that's completely unexpected or impossible—yet looks completely real? Perhaps you want to take a more subtle approach, seamlessly blending subject and scene in such a way that the viewer has to stop and consider what he or she is seeing, and whether or not it's real? Each of these approaches (and every variation in between) can be a valid and worthwhile endeavor.
As long as the art you're creating means something to you on a personal level, and as long as it's executed with forethought and attention to detail, you're likely to create a successful work of digital art. There are no formulas. Give yourself plenty of options and explore them before you begin the project.
As when a painter or illustrator is working on a new concept, sometimes it's best to sketch out many possible subjects and scenes on paper or with your digital stylus. Getting a rough 3D visualization of what your finished composite might look like can be enough for you to see right away that it can't work—or that it's the perfect idea.