Looks good already!
Here is what I'd do:
Assuming that you want to keep most of his armor black you need other things to make the model catchy. From my experience especially at GW painting competitions not necessarily the best painted model wins but the one that catches most attention.
As a general rule of thumb you should always decide which point of the model you want a spectator to look at first, I usually recommend the head. In order to achieve that the head needs to contrast with the rest of the model. In your example I'd take the bone-runes on the helmet one step brighter and do something about the eyes to make them stand out more. Maybe try some OSL (Object Source Lighting)
Next up would be the red gems, as they are they look a bit unnatural. Try to google for a reference pic or if you have any jewelry take that for reference. Decide on a direction you want the light to come from and highlight just the outer parts of the gem in the opposite direction. Say you have the light coming directly from above, highlight the low parts gradually. once you're done take pure white to add a glimmer spot in the direction the light does come from to represent the reflections.
If you need more advice or any clarification of my ramblings feel free to ask.