Personally, I wouldn't want to see any substantial change to the building at all. Now that it's built and done, I really like it's moody-grey massing, just the way it is.
I believe there was a plan to have a glass overhang along the length of the entire south side - I'd like to see that go in, though.
The only other thing that I think would be beneficial is, again, the opening of the north-east corner at ground level to public use to animate the space. The car ads look desperate, and are a downer.
The ground level hallway that connects the lobby to this space is currently pretty much blocked off, and the glimpses you get of the interior here is that it's pretty much a utilitarian hallway. This is the one bad dead spot in the design, IMO. I wonder if something could be done to remedy that? Polish up the hallway inside, and connect the glass along the ground level from east to west. It would be a slight change, but a beneficial one, I think.
I suppose those utilitarian hallways have to go somewhere, but I also can't help but think Mr. Diamond might have made better use of it if he'd had more money (or certainly, a bigger lot).
Some of the blame here has to go to the Sheraton Centre across the way. If it was only this corner of the Opera House that was unengaging, it would be no big deal. But faced with the concrete blanks of the Sheraton across the street, the cumulative effect is bad for enjoyment of the street.