The habitable modules of US sector of the ISS didn't have any propulsion systems to rendezvous the ISS by themselves. They were intended to be brought there by the Space Shuttle. Most of the Russian heavy modules of Mir and the ISS have arrived to the stations by themselves.
To bring 11 tons Kvant-1 module to Mir, Russia has used the disposable FSM space tug, based on TKS.
Has NASA announced any realistic space tug projects to be able to bring a habitable module, such as, say, Node 4 or a similar payload to the ISS?