Kargath bladefist is cool, and could definitely use some love. The nice thing is that you could make both versions, since you could easily just reskin it (or have it use fel orc skins) if you need a fel-orc version.
Someone made a pretty quality rend blackhand already (
link) so IMO you should do something else.
Kil'rogg has some distinct features that make him a cool concept to work with.
Ner'zhul is a very prominent figure, but his main distinction is his mask--it might look like a standard orc apart from that.
Gul'dan is just plain cool. You could really do a lot with it, and he is a pretty major figure. Tauer already made a pretty good interpretation of his wc2 model, but no one has made a good version of his WoW model/design.
I wouldn't make grom, I personally love his current wc3 model. It was my favorite model ever since I first played the game. I desperately wanted to play him during the RoC orc campaign several years ago, and I was so happy when we got to the level with gathering lumber & the cenarius level.
And.... the weird thing about Durotan is that there isn't much info on how he looks (iirc). A lot of it is open to interpretation, gear-wise and all.
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In my personal opinion, Gul'dan would look the coolest. Good luck.
