I think the original expansion plan was to fill in the grass with seats before starting on a second layer of seats on the sides. That being said, people have really grown to like the grass at the end zones. A temporary situation has become a feature so I don't know if they will fill those in now. There was also discussion a while back about a sizable donation being made towards a new team center.
I’d say filling in the north end zone, opposite the video screen would be logical. Concrete, rebar, benches and hand rails. It would also preserve the berm with moving shade from the screen for families. Depends on the footprint for future athletics facilities though - that may alter that idea.
Keep the grass for now. A training facility overlooking the field would be cool, a second or third level could include a banquet room / viewing balcony / deck thar could be used by many.
An upper class or grad student in environmental design or such put together a thorough study and proposal a number of years ago on how to tweak and improve AS, including some options for shade.
As far as cementing in the end zone, I'm not sure we'd gain more than we would lose. Say a thousand people sit in the grass, and cement gets us 2,000 poor-sight seats... that's not a big bump.
A double deck over the student section could add 4-8,000 seats (realistically), would add shade for the lower level, and probably would add restrooms and additional concessions. Maybe even reception boxes and great sight lines.