The Energia/Buran was pretty much a TSTO craft. Energia is a single stage rocket that can carry payload either on top OR stacked on the side. We should build our own Energia launcher, then design a new second generation shuttle with a metal TPS. Such a launcher would be able to lift large single module space stations, the shuttle, lunar missions, as well as Mars missions. One common launch system to unite the separate components of a robust space program.

One launcher to carry them all
One launcher to lift them
One launcher to bring them all
and in the darkness combine them
In the Great Void where our destiny lies.