Viola Davis was the first name to come to mind for me too, but at the same time I think she's almost too strong for the role, she would sweep the floor with Will Smith, frankly it's hard to think of someone who could match her. I haven't seen FENCES but it seems like Denzel was able to do well opposite her, so more reason to cast her as Stanley. Smith is simply an awful choice for the role.
And by the way, CAT ON A HOT TIN ROOF could have worked wonderfully if only the cast hadn't been led by an awfully awkward Terrence Howard and a misguided James Earl Jones. Anika Noni Rose was a brilliant Maggie, I'll never forget the experience of watching her carry the play on her shoulders, she also swept the floor with Howard which was a shame.
For the record, Jada was offered the role of Stella, not Blanche.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"