Another idea of API method :
Within the selected area, generated some randomness in land elevation. Something like
world.setTerrainRandomHeight(int ?, int ?, int ?, byte ?, byte ?, byte ?, int ?, int ?, int ?, byte ?, byte ?, byte ?, byte sharpness)
Where the sharpness variable indicates how steep the slopes should be (average), from 0 (flat) to 100 (vertical "walls")
Or, perhaps making this method into a slope generator, with randomness. Something like
world.setSlopeTerrainDataInArea(int ?, int ?, int ?, byte ?, byte ?, byte ?, int ?, int ?, int ?, byte ?, byte ?, byte ?, byte sharpness, byte orientation)
Where orientation is a value from 0 to 4 indicating the direction of the slope.