You have some really cool designs and a good imagination! In my opinion, I think it would help to practice your lineart to help define form and structure. For example, your two sci-fi/steampunk cyborg characters (underneath the cityscape) have nice silhouettes and colors. But they looked a little muddy, and I couldn't always tell what their costume was made of.
Remember that concept art is not about how well it's painted - what's important is how clear the design is, and that the design itself is interesting to look at. Keep it up, hope to see more from you!