Not so easy. Hugo made $185m worldwide and still lost money. Bringing out the Dead and Kundun were flops (financially). Basically, at this point Leo is the only reason most of Scorsese's films are getting financed, and in fact Wolf of Wall Street was Leo's baby (he pretty much bought the rights and recruited Scorsese to film it, similar to what happened with The Aviator).
If Scorsese wanted to make the expensive films he does without casting Leo, he could probably only cast someone comparatively big like Depp, Clooney, Tom Cruise, Will Smith, or Brad Pitt.