I imagine anyone with any amount of urban awareness has had their own ideas of what to do with this space as soon as they first laid eyes on it. It's clear the first time you visit that the plaza is just not working right. Crowds choke the area around the cable car turnaround and pedestrians avoid the relatively broad walkways along the old Bank of America building and along Market Street. BART and Muni riders hurriedly pass through the sunken area in the middle, and the area West of Cyril Magnin is an afterthought if it's thought of at all.
And any amateur urbanist's bright ideas for the plaza would probably be an improvement. The plan discussed in Gordon's article, which is apparently being seriously considered by the city, calls for that Western segment to be decked over and converted to a reservoir. That's a very practical use of the existing volume, and it would improve the plaza, to be sure. One disadvantage is that the plan would cut off access to the Powell Street Station from West of 5th Street. Riders coming from, say, the Warfield theater would need to cross either Market of 5th Street to get underground.
I confess to having my own daydreams of what Hallidie Plaza should look like. I can't say I've ironed out the details yet but in response to Gordon's article I'd like to mention some things I think are important to consider in any overhaul plans, vaguely in order of importance.