Can't imagine they'll ever really be able to find a big name for Natasha. Vocally, the role is extremely demanding, requiring a high soprano and a strong belt, and when you add in the physical nature of the production (all those stairs!) it's really a beast. Sonya or Helene seem like much likelier opportunities for star casting- fun roles with a plum song each, and not nearly as challenging to sing or act.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.