With the Rapid Ride Green Line starting on August 15, I think now is the time to speculate on where you would put a fourth Rapid Ride route.
I know that a San Mateo route has been considered in the past. My idea of such route would start at Jefferson and Osuna where the 141 starts, would go south on San Mateo to Gibson, and would go east on Gibson into Kirtland AFB. I think there is definately enough demand for another route into Kirtland AFB, considering how packed the 96 and the 317 get.
The route would be known as the Rapid Ride Route 740 Pink Line. While some may argue that pink would be indistinguishable from red, perhaps a light shade of pink can be used to make the route more identifiable.