I have also been contemplating the whole INQ28 thing and put together a few models. I transformed a chaos cultist into a inquisitor retinue type heavy gunner (middle). I decided to paint these models different from what I normall do, which you can see down below.
I prime the model black and then use a light spray from one side. All the parts that are covered by the light spray, is what I will actually paint, the rest stays black or very muted. That way you get the shading to make it look a bit more dramatic.
Obviously, the witch hunter isn't done by a long stretch, but you kind of get the idea. You can see I added an empire wizard hand to make him a bit more myserious.