What if we could say, without laughing, that the Outer Richmond was the new Mission? That would only really have been possible if BART had gotten it together to build the proposed Geary Line of the system, which would have branched off at Montgomery and stopped at Union Square, Van Ness, Fillmore, Masonic, Arguello, 11th Ave., and 25th Ave. Imagine!
Of course, much like this wildly ambitious, dreamy plan for a BART system that served the whole city and the whole Bay Area, Napa and Sonoma included, the Geary Line was but a pipe dream. The above plan, dug up by transit archivist Eric Fischer, is from 1961, at which point the proposed Marin line had already been nixed. See all of his collected transit plans and maps here.
The real, actuated BART plan, like all things, was subject to various limitations and cost-cutting measures, and BART as we know it today only serves a small segment of the city, making places like the Richmond feel in the minds of those who don't live there about as far away as downtown Oakland, which it basically is, time-wise.