OK, though I am usually "in the know" about various fantasy and sci fi upcoming films, I have no knowledge of any plans for a "Den" film, or if there has been any plans in the past. There for sure must have been talk of it over the decades.
I collected Heavy Metal throughout my teens, and Den was my favorite. The epic tale of a young geek who got swept from his suburban back yard, transferred into a muscular body, and taken to a grim/dark fantasy world much as his uncle had once been, is a D&D fans delight, or even just a fan of strange fantasy. I loved seeing Den in animated form in the Heavy Metal film of the 80's.
OK, I'm not a Vin Diesel hater. I liked him in Pitch Black. Though I haven't watched the films he is best know for (outside of hearing his voice in Guardians of The Galaxy), I respect his geek cred. He is an unashamed D&D gamer, and also has a great love of video games. But the casting of Vin would make the movie more about him than the heroic Den himself.
But dang. Despite maybe being a good couple decades or so older than the young Den seems to be, he just fits the part physically, and probably in personality as well.
I'm sure Vin will have a long and healthy life. So by the time they do a Den film he'll be too old, but will at least see it (surely Vin has seen his resemblance the character. He's a geek). And hey: Den actually encountered his buff uncle at some point in the stories, so he ain't out of the running of a film I hope I'm around to see when its made.
Dare to dream!