I think you have a solid base to work with. So I have a few suggestions that hopefully will help you. I did a rough paintover to better explain it.
So first thing I'd expand the canvas a little bit at the bottom so that batman doesn't seem like he's standing on the edge of the canvas. You generally want to avoid that kind of stuff.
Also, if you want to keep the background as a simple gradient I'd recommend painting it yourself instead of using the default gradient tool. I will generally look more interesting and you can play with textures and color variations more easily.
If you feel more courageous, you could also add a setting like a city silhouette or something like that in the background. It could help give more depth to your piece.
Next, I think you can be bolder with your shadows. By that, I mean to not be afraid to define them more and make them darker. Right now they look a bit too undefined. I think it will help make him pop out more.
This change is subtle, but I think it helps makes batman blend better with the picture. I started by shifting the color balance of batman more towards the reds for the lighter colors and midtones. Since the background is filled with red, it seemed like it should affect the colors of his costume. At least it would if you go for a more realistic look.
And then, I also shaded the bottom part of the body darker as light would normally not reach as strongly the lower part of the body as it would the upper part. And by exaggerating the effect it helps give him more weight.
And lastly, I added an edge light on the left to help separate him from the background and make the silhouette read better. I chose a cooler color for it to for constrast and color variation.
Beyond that, maybe you could try to find references for the legs. There's some interesting shapes that the muscles do that I feel you haven't quite captured. I didn't attempt to correct them, because I struggle a lot with legs so I don't feel like I could do it right 
So yeah, I think you can do some cool stuff with this piece. I hope my feedback was helpful in some way or that it at least gave you a few ideas 
