Latest update:

First off, thanks for your interest and thanks for doing the edit! Now, to answer your comments:
Lacking skulls and spikes:
This is still at a very early stage, which is why the shoulderpad is so clean, for example. There's a lot of stuff I haven't added yet. I prefer to draw in layers, so I'll draw most of the armour and then draw over it with decoration, skulls, spikes, dents, bullet holes etc, in time.
Re: Why not put the axe in the other hand?
Well, one of the interesting features of Khârn is the fact that he's mysteriously ripped off the left shoulderpad and arm off his power armour and is carrying a huge fricking chain-axe (!) called Gorechild in the same hand. As far as I know, there's no official explanation why. Maybe he wanted to strip his armour of his Legion markings, as an expression to his loyalty to Khorne above all else. He is the betrayer, after all. Regardless, there's a nice contrast between the barbaric element of his naked arm and the futuristic weapon he's holding. It's quite iconic, I think.
You'll also note that I left his right hand empty, instead of having him hold his trademark Giger-esque master-crafted plasma pistol, because I wanted to copy the pose in the reference I posted above, showing a Chaos Space Marine with a power fist and a chain sword. I do think that the original artwork by Mark Gibbons is better and more intimidating, because the Marine is looking right at the viewer, whereas I have Khârn looking slightly to the left. BUT... I feel like swapping the chain axe to the right hand makes the whole portrait a bit unbalanced, because he's looking to the right, the weapon is in his right hand, he's got a huge shoulder pad on the right side, while his left side would be naked with a small ball and chain. I could put the plasma pistol in his left hand, with some glowing light and smoke to add interest to the right side of the portrait, but... I'll have to think about it.

There is one thing I liked immensely about your edit was the green tassle fluttering in the wind. Because nothing says badass like a statuesque pose in turbulent surroundings, standing still while there's a raging storm or battle going on in the background. I did consider adding a ripped cloth, flag or banner in his clenched fist, fluttering in the wind, but I played around with it and couldn't make it look good. For now, I will at least make the tassles a bit more dynamic. At least that's something

I very much looking forward to seeing your Plaguemarine (?), there's so much stuff a creative person can do with them.
If anyone has the time or desire to help me further, I would especially appreciate technical feedback regarding what I have so far. Does anything look wrong, is anything out of proportion, is the pose wrong, should I change the colours, is my pixelling technique off, have I pixelled the metal realistically, does the axe look a bit weird... those sorts of things. At this late-ish stage, I'm not primarily looking for ideas like changing the equipment or pose, because... well, I've already put some time into this and I'm more worried about fixing mistakes or improving major flaws right now.