I think in DAF the producers have replaced them with Blofeld and Spectre because of the previous films. The Spangs are always regarded as little impressive villains. But for the screenwriters that would not be the big problem, because Le Chiffre and Goldfinger were IMO in the novels also not as impressive as in the films.
I think the producers have two major problems with the Spangs. On the one hand they are a villainous duo (not the one big bad guy) and on the other hand the producers probably never liked the idea that Bond travels to the US and fights in the Dashiell Hammett & Raymond Chandler style against american mobsters. In addition, the Spangs would need a modern and really threatening plan. Diamond smuggling is not enough.
All of these problems are certainly solvable. The characters should be upgraded, expanded and more complex. Bond against a psychopathic brother duo could become a very exciting constellation, because the Spangs other than Khan and Orlov in OP for example are not pure business partners, but compete as a family clan against 007. Since there is much more dramaturgical potential.
The only point that is really difficult is the big, evil plan that the Spangs should pursue. What should that be? Yes, of course you could use the old GF plot with the resource shortage in a certain way (diamonds instead of gold), but that leads to a series of difficulties. As a matter of fact, most of the diamonds are won in Africa and especially in South Africa. So, Bond and the Spangs would also have to settle the conflict there. But how do you get them back to the United States? Why should 007 just meet Leiter here? I think the charm is somehow in the fact that Bond is fighting the Spangs in their own sphere of influence.
I think the glittering world of Las Vegas and the Death Valley are still exciting and great locations. Specifically, Spectreville and the following train chase are a very exciting and atmospherically dense passage in the novel. In the 1971 film, I found the Vegas and desert scenes basically atmospherically implemented, but overall I see there still much potential. We had already several times Venice, Istanbul and the Bahamas, why should Bond not be a second time in Las Vegas? As I said, the only problematic thing is the great operation that 007 has to prevent. Perhaps it would be wise for the producers to throw the entire diamond plot out. This was also used in the 1971 fim so that the writers could choose a fresh, new plot approach.
