Actually, I think Ultimate Marvel Knights is still a workable concept, as compound and I have been toying with the subject for quite a few months now.
I think for it to be patently interesting would take a kind of re-imagination that might scream heresy for some devotees of the characters. I mentioned it elsewhere, in fact on one of MWOF's DD threads but didn't really go into details.
The thing is, many of the Silver Age Marvel characters who have now been Ultimatized --- The Fantastic Four, The X-Men and The Avengers --- were born out of anxiety over the atomic bomb and radiation. Ultimate characters tend to reflect an anxiety over power, over being more than human, reflecting a kind of (wait for it) superhuman zeitgeist.
But the Knights characters who were born in the 70s-90s cared little for the kind of technological anxiety that the Silver Age characters reflected. Luke Cage was an effort to reflect the African-American unrest and the trend in media towards blaxploitation. Ghost Rider refracted the 70's spiritual anxiety back to itself, at a time when America felt that the 60s idealism petered out into social and moral decay. Frank Miller's work on Daredevil explained how an individual can transcend their impoverished upbringing and their handicaps and stave off urban despair.
For an Ultimate Knights franchise to work, the characters cannot just exist alongside the existing Ultimate characters for their own sake. They have to be reexamined thematically and see their own themes "modernized".
How does a man with strong social ties, to his family and to his faith suddenly decide to go armed to the teeth on an antisocial crusade against crime? What is family and what is war? Where does punishment end and vengeance begin?
What is the meaning of damnation and salvation at a time when, post 9/11, the difference between the Judaeo-Christian, the Islamic and the agnostic is at its most tense? How do you summon up the worst in you and turn your own spirits of vengeance into a force of justice?
Why does a man, set back by the pain of a handicap, the injustice of the city, the loss of a father never lose hope? How does he continuously face failure and disappointment, suffer the circles of violence and pain and remain fearless?
How does one man face a world so compromised, drive a stake through the heart of evil, knowing that his accomplishments shall never be acknowledged? How does he take a blade to the darkness and never lose sight of what is black and what is white?
To examine that in ways the Marvel Knights never did, the Ultimate Knights have to be more different from their 616 predecessors than any other Ultimate characters.
Ultimate Spider-Man took the famous oath of "power and responsbility" and changed it to become a story of a boy learning to become a man. Ultimate X-Men reimagined the sociopolitical struggle into a question of ethics. The Ultimates took the ever-changing nature of a peacekeeping force like the Avengers and examines the tenuous nature of alliances between people, institutions and each other.
For Ultimate Knights to just exist, unchanged is doing an injustice to the ambitions of the existing Ultimate titles.