Loved LA Noire, loved Mafia even more. But I can't see how a cross over between these two would be great especially with a massive online component.
Here human controlled mafia player, sit there while I interrogate you for 5 minutes.
Well, maybe it's two games in one? You can play as a mafia man and build up your crew, or you can play as a cop and try to clean up the streets. So one game world, one game engine, but two completely different ways to play.
(The LA Noire scenario portion feels more like DLC than a full second game playthrough compared to a full-fledged Mafia title, but maybe if Rockstar is less concerned with all the highfalutin yoke that Bondi focused on in the first game and instead the new team focused on things that complimented/contrasted a Mafia game using that technology and Rockstar mechanics, maybe there's more to it than what the first LA Noire had to offer, something that could avoid the deficiencies of the first game and still capitalize on what people liked of Noire.)
GTA 5's character switching but you'll be switching between a cop and a gangster.
...What he said (Should have read the thread.) Impossible to say if Rockstar sees a future in the character-switching mechanic (GTA5 was a just plain juggernaut, you can't say whether that mechanic sold more copies or not; the developers do seem happy they pulled it off, but who knows if this is the way forward or just a way of refreshing the series for the team,) but they do have something to learn from in making such a game if they do.
The problem with your version of the idea is, it'd be weird to be switching between a good guy and a bad guy. In GTA, you had the same goals, but it'd be hard to imagine a scenario where both the cops (and we're talking hard-boiled noir cops, ones who on any given day may rule or get rolled on the streets, not just the average on-the-dole cops, otherwise it's not Noire) and the crooks would tag-team for the same agenda. I don't see the mafia having much concern for a serial killer or something like that which would keep cops out, and even a gang war that a cop might have to pick the Mafia's side on in order to beat back the greater evil, I still don't see them going about the same missions at the same time for the same ends. It'd be cool if they could find a way to cross them up organically, but a way of doing it is not coming to mind.