Here is a two-wheeled vehicle that can negotiate almost any terrain and fit in a suitcase. It has a 4-stroke engine and can reach speeds up to 60 km/h. You get a 3 speed transmission system, 220mm wide wheels, and a steering angle of 55 degrees. The below video shows the Taurus 2M ATV in action:
As you can see in the above video, this is not your classic foldable bike, but you can take it apart to fit it in a couple of bags. You can find out more about it here.