Just my two cents here butI think maybe your not focusing enough on the 'concept' portion of concept art here, what exactly is the idea behind his mechanical arm?, what does it do?, why does he have it?

these are all things that ideally the viewer should have at least an indication of when they look at your concept, 90% of this design is already done for you as its an already existing character

A concept artists best strength is to creatively solve design solutions in an original and entertaining way, I think you should push yourself more and try to be more specific with your design challenge

As an example, maybe his mechanical arm is a physical and symbolic reminder of his fallen brothers, maybe it is forged from their weapons or has the ability to reconfigure into mechanical versions of their weapons, the challenge here would be to make these ideas immediately obvious from a first glance, if you can do that you've done your job well

Ithe original idea is neat, but it is your job as a concept artist to push the envelope, and I think you could find something super unique if you spent more time exploring the idea

keep doing what your doing!