First of all, I'm pretty sure this blank wall won't be exposed but even if it were, there are examples of exposed blank walls all over town and some of them are very interesting. Right across the street, I believe, at 560 Mission, a blank wall of the Golden Gate U. building got exposed and they covered it with tall bamboo--looks great. At 199 Fremont, blank walls were also covered with a screen of vegitation to creat a very nice minipark. It's no big deal.
Also, I happen to think one thing that adds a huge amount of interest to downtown San Francisco is the mix of modern and older structures. Usually, the older buildings have lower rents and so are commercially viable for things like lower-priced restaurants that simply couldn't exist if the only places they had to locate were modern highrises. Prior to the demolition for 555 Mission, there was a very popular cafeteria-style Chinese place on the lot and I missed it when it closed. I am quite sure nothing like that would ever occupy space at 555 and, unfortunately, the little remaining building you're worried about probably isn't big enough, but maybe something to relieve the hard and relatively uninteresting lobbies (unless you work there) of highrises on the block will go in there.