That'd be nice, but Geary is not really a multi-lane road until you go west of Van Ness, and the main areas of congestion seem to be downtown, east of Van Ness.
The way the current metro light-rail is laid out is a good idea on paper: underground for the dense urban areas, and above ground where you can have space for a dedicated lane. If a potential Geary Metro was underground from Embarcadero to Japantown (Laguna or Webster), then above ground the rest of the way, I can see see a light-rail system working. Now let's find a politician with the clout to have Geary under construction for 5-7 years (optimistically).
East of Van Ness, Geary and O'Farrell form a one-way pair. But yes, there's not much space, and the area dense so running underground would make sense.
As others have pointed out, there is in fact a BRT project in progress to bring a separated, dedicated-lane busway to Geary [1].
The way the current metro light-rail is laid out is a good idea on paper: underground for the dense urban areas, and above ground where you can have space for a dedicated lane. If a potential Geary Metro was underground from Embarcadero to Japantown (Laguna or Webster), then above ground the rest of the way, I can see see a light-rail system working. Now let's find a politician with the clout to have Geary under construction for 5-7 years (optimistically).