Just an idea. Sometimes we want to extrude something that will be permanently bonded to another object. Now, we'd need a new "head" for this. First, we place the object on the build area. Then we scan the object (laser, touch sensor, camera, whatever). We'd need an STL generated from this point cloud. Then we design over top of it, effectively boolean subtracting the object from the final part. Then we'd print right on top of the existing unit, forming a solid connection.
Now, I do see issues mechanically (can't print "lips", warping, nose drips), but that is a case by case basis or part of the reprap mechanical improvements from generation to generation. I'm ignoring the point cloud sensor head and software conversion. There are other people working on systems like that out in the world.
Software-wise we'd need to be able to raise the print head so it wouldn't run into the existing structure below. This would require some additional intelligence from the routine generating software, but I don't think too much.
