Even though the Central Subway still isn't due to be completed until early next year, the SFMTA has a public survey in which its gathering input about how the subway line might get extended. Want to take a train from Union Square to the Presidio? That could be an option. Someday.
Planning for the Central Subway has long considered various options for extending the train line — and a study was commissioned in 2014 to look at various options for new station stops along the line, in anticipation of the city's doomed bid for the 2024 Summer Olympics. As it stands, the subway tunnel that was dug already extends to North Beach, where the boring machines were ultimately extracted, so adding a North Beach stop is a no-brainer. And there's always been talk of going at least another stop further than that, to Fisherman's Wharf.
But Curbed points us to this survey just posted by the SFMTA, in which its looking for public input both about transit priorities (no surprise: the highest ranked priority so far is "transit reliability"), and about where and why to extend the Central Subway. It's quick to fill out, and it asks you to pinpoint where you live and work, and to indicate why a Presidio or Marina stop would be useful to you — you can just say it's "near neighborhood amenities" or "near tourist attractions" if you don't live or work near there.
Obviously, having a way to get to the Marina that didn't involve a bus would be a happy thing for everyone, but imagine getting all the way to the Presidio? Underground? You could also just throw in stations all the way along Geary if you wanted. The map is yours to play with, no matter how unrealistic your desires might be.
And as for the existing part of the Central Subway, it may be finished by February, but don't expect to be riding any trains on the line until later in the year. Maybe a lot later, because: Muni.