Though really, I always considered CASINO to be a complementary, "side-story" true-story mafia tale set in the same universe of GOODFELLAS, but without any connections. In a way, its like FLAGS OF OUR FATHERS and LETTERS FROM IWO JIMA were sold in 2006.
Anyway, the reason why DeNiro and Scorsese haven't worked again is different reasons. DeNiro has gradually settled into more of a film producer, making millions from those MEET comedies(and probably even more if they go ahead with #3) so he can produce movies personal to him(i.e. THE GOOD SHEPERD), and to his Tribeca film festival. I wish DeNiro would kickass acting-wise again in a film, but hey, its his career, his choice.
Of course, I thought THE GOOD SHEPHERD was a mess, but its obviously a personal project of his so I can't get too pissed about it(or maybe I can. I still think DeNiro blew a great opportunity to tell the real slam dunk dirty expose drama of the CIA's nasty work in the 1950s like Iran, Guatemala, you name it!)
Anyway, Scorsese meanwhile is off trying to get his Oscar or at least continue to direct his documentaries and whatever projects that come to him. Hell, he and DeNiro came close a few times of working together again.
GANGS OF NEW YORK in the late 1970s was going to have DeNiro play Bill the Butcher against The Clash's Joel Strummer as Amsterdam. By 2002, DeNiro lost interest(and too old) and Strummer was dead.
Scorsese tried to interest DeNiro with BRINGING OUT THE DEAD, but DeNiro politely passed on it(and I don't blame him. Its basically a retread of TAXI DRIVER). DeNiro also tried and failed to secure some sort of film part within Scorsese's biggest box-office hit, THE DEPARTED(if I remember right, the Martin Sheen and Jack Nicholson roles could have been his).
Perhaps the biggest problem so far with making another DeNiro-Scorsese reunion is the fact that Scorsese has found a new frequent lead male star that's become a pupil under his watch: Leonardo DiCaprio. Certainly Leo Boy had raw potential talent as an actor, but he's blossomed with Scorsese in just 3 movies so far. Besides, he increases Scorsese's financial viability within Hollywood for the next good number of years.
Anyway, DeNiro and Scorsese are possible to work together again. There has been that "childhood neighborhood" script that Scorsese has personally been toying with for decades that DeNiro wants in with, and of course THE DEPARTED sequel where DeNiro would probably be the baddie to survivor Mark Whalberg(that is, if Scorsese decides to direct it).
Interestingly, you all know that DeNiro and Scorsese in 1995 were supposed to make CLOCKERS instead of CASINO? It was going to be a police procedural tale, but they lost interest, and Scorsese got enraptured with CASINO instead. He stayed as a producer, where CLOCKERS was retooled as a solid gangbanger drama from Spike Lee.
