I love how you incorporated Mucha's style and your own work! Beautiful! (And ver glad you didn't make him a woman in traditional taste) lol <3 <3 Using his cape instead of flowing hair is genius. *gush gush*
Some people just put a decorative frame on things and call it Art Nouveau, but this is real art. His pose and lines and curves echo and play with the lines and curves of the frame, the color values echo each other, and the leafless tree is a perfect symbol of his existence - while the irregular shapes and dark color of man and tree contrast with the perfection of lines and curves, and their pure whiteness, which to me is also a contrast between the imperfect man and world and the inhuman perfection of his dream of Justice, as embodied in the Batman's purpose.
No, I don't particularly hunt for symbolism in art; it's just that the harmonious contrasts of your picture make it, hmm, active rather than static. Art rather than icon (in the older sense of icon).