I like the overall look, simplicity is not necessarily a bad thing - better than over the top. I'm not sure what program you're creating this in, but if you can make gradients, you can add some depth to the design without changing it much.
For the rook, you could use a left-to-right linear gradient, meaning just a fade from one color to another. If go from a brown that's a little lighter (than what you have) on the left, to a brown that's a little darker on the right, that would make it pop a little. For the main part of the shield (the light brown, not the red), you could do the same, again taking colors a little lighter and darker than what you have, and creating a gradient either from the top to bottom, or upper left to lower right. It's a relatively simple way to make it look a little less flat, while keeping things close to your original.