I think Amtrak could see the most success by building out to support the cash cow that is the Northeast corridor. That would mean improving service on the Northeast corridor, to improve speeds for Acela, and adding additional higher speed service to connecting cities.
Two good targets for this would be Upstate NY, where you could reduce travel time to Buffalo from 8 hours to 5 hours, and make travel to Chicago and Toronto much less painful. Improving the Philadelphia -- Pittsburgh route would also make a lot of sense too.