Ok brah dont hate, but the sig is sh1t ( this is only an advice )
Nothing blends with the background.
Change the background, dont cramp all Assassins in one pic, even though they're assassins they all have different stories.
Add some C4D, download the Resource pack for them, you can find a resource pack the design hall resources and tuts.
Add fractals, add bubbles, add gradient, add topaz, add a lot of things man.
Use a light resource you can create this by creating a new layer, take a soft brush brush white ( or whatever color you like) on the top of the image, change the blending option into soft light.
Download brushes such as Nebula ( hi Nebi ) and add some space effect on that image.
And many other things bro, just search sig tuts on devianart and you'll learn.
Best of luck to you man