So what are they talking about?
Maria Callas had a big voice with lots of squillo and a burning ambition. From deep in her soul she wanted to be a star. She worked very hard to develop a proper bel canto technique and keep the flexibility in her voice. She was both naturally expressive and completely obsessive. She cared about expression and developed her interpretations with great care. Her voice, her character, her technique, her expression, her ambition all combined to make a very great artist. That's why it was so shocking when she suddenly abandoned opera for mere celebrity.
So every dark voice with a bit of coloratura is "the next Maria Callas."
The route to greatness, the route followed by Maria herself, lies in beating ones own path, in making music that is completely ones own, in defying comparison. No artist succeeds by imitation.
When I look around at the singers today, only Angela and Cecilia seem to burn for it. Cecilia's gifts don't lie in Callas's spinto repertoire. I haven't always liked what she was doing, but she more than anyone working today is blazing her own trail, creating performances of true originality. Her creativity is astounding.
Anna does not burn for it. She would never have backed down on her bid for Austrian citizenship if she did.
When they say someone is the "next Maria Callas," they're just wishing. They want to sit in a hall and feel their spine tingling, their soul shaking as it once did with Callas. We want to be thrilled and not merely entertained.