Galactic engineering is focused on modifying the large scale structure of the galaxy. In the natural universe, such modifications result, for example, when galaxies collide. However, there is on reason humans couldn't eventually undertake engineering at thé scale of the galaxy. This might involve modifying the speed of rotation of the spiral density wave, increasing or decreasing the size of the central bulge, increasing thé metals content of large patches of the galaxy, and so on.