Your poster designs are very clean and memorable, the only problem I notice with them is a large amount of negative space. The Robocop one is the best (I think so) because he is framed by the fire. This gives the image, while not a symetrical look, a uniform look. Both sides of the image have the same amount of content.
Think about how you could frame your other pieces and fill in that negative space. The frame doesn't need to be so well worked into the image. It could literally be a frame around the imgae. It could look like the paper was burned along the edges. You could use small plumes of smoke. You could even frame the image with shadow and center all of the light only on the main subject. Play around with some of your finished designs and se what works and what doesn't.
