I've seen all of that artist's recreations on DeviantArt
I'm actually not a huge fan of many of them for being DOOM demons but I love them as their own monsters.
Here's http://www.deviantart.com/art/Let-s-do-Doom-Pt2-Archvile-339674204 for example. I love it, but not as an Archvile, I simply can't think "archvile" when I look at it, or DOOM. Though if I were a game studio I'd hire the guy right away, he can make some incredible monsters
What I find to the be greatest pain elemental ever designed is http://www.spriters-resource.com/resources/sheets/35/37573.png, I've never seen a more frightening one. Too bad no one has ever made a decent fan art for it, they always look goofy.
Well, the Doom wiki shows suppsed concept art that does include futuristic mechasoldiers that could easily take out the smaller demons.
If they really had thought of it, it shows how sad it is that they played it safe with Doom3