I think the plaza being below street level like that is unworkable. People don't gravitate toward grade changes like that without a destination, and without anchor tenants nobody's making it a destination. Maybe if they improved the sight lines from the sidewalk to the retail storefronts there'd be a chance.
Maybe what they're going for with this glass structure talk is something like the 5th Avenue Apple Store, where at street level all you see is this:
And the real store is below:
Making an attractive storefront that captures millions of visitors while dealing with an awkward physical footprint yet keeps the street level. They could kind of dig out the Hancock plaza and put anything down there, and build out a platform over it that behaves like a street level plaza. Who knows.