GW really doesn't like any of their named characters being used in anything they don't do themselves; it's why, say, Vect had such a small role in his one video game appearance, and why you'll never get Eldrad or anyone like him appearing anywhere. That kinda makes it hard for some people.
GW does, however, let people renting their license come up with new characters, which really did work for DoW and will probably do loads of good for DNA depending upon how they run their story. Boom Studios might be able to get something going, too.
Emperor knows there's enough lore to draw upon and tell original stories or do something cool with.