There something about a lawn at the foot of a tall landmark that just fits.
I'd love to see this incorporated into the East side of the property, perhaps as a set of green ripples (hills) propagating from the tower. This would be reminiscent of HTO Park and the wave decks that are becoming the "brand" for our waterfront.
I don't know if many people would use it. There's a huge park around roundhouse and I seldom see people hanging out over there. Most people either hang out at the brewery or walking around at the sky dome, CN tower area.
Roundhouse Park has never been quite the hospitable park, nor has the area around it.
Things are changing on Bremner, the Roundhouse and the Park are getting spruced up. All you need is a well designed park (i.e. HTO) and people will want to hang around.
I would think that the most frequent users of the plaza (aside from tourists) would be sports fans doing some Toronto-style sidewalk tailgating before going into Skydome. I'd like to see SkyDome (or Rogers, which owns the stadium) get involved in this development. The stairs leading from the John Street bridge down to the current plaza is very boring. I would like to see that replaced by something more inviting to both locals and tourists.
They could integrate the stairs with the salmon pond water feature into a water fall of sorts with the steps on either side of the water. Somewhat similar to what's done on the West side.