There doesn't seem to be any appetite whatsoever for filling in more of the San Francisco Bay to create land, and there are restrictions and conservation easements on most of the land stretching from 280 to the Coastside.
One possible area to expand into would be Coyote Valley, south of San Jose, which was a growth target during the first dot.com bubble in 1999, around a Cisco campus. If Caltrain could put in a station there, along with express lines, that may open up a middle-class area to new housing opportunities.