The Tomorrowland Terrace really is a very undutilized space, considering its location and views. It would be great if they could find a way to utilize that space for a variety of needs.
An astronomy-themed restaurant would be perfect, if they can find a way to continue to utilize the castle views. The concept to me sounds like something that would require a dark, closed atmosphere, and I hope they wouldn't wall off that view completely. Especially given that places with an ability to view the fireworks are in such high demand.
Considering how Disney often likes to share a single kitchen across multiple venues, it would be great if they could open an astronomy-themed, all-indoors TSR, with a small QSR terrace to try and offset the demand at Cosmic Ray's (like how the waterfront QSR and TSR restaurants at Mexico share a kitchen). Then the QSR could shut down to operate as a viewing terrace for the TSR once the fireworks start.
The rooftop viewing deck is a great idea also, since it would even further increase capacity, and wouldn't require anything to shut down if Disney wanted to host private events.