If it were possible for me to change the human brain, I would start off by altering the overall structure of it. Instead of being divided into two hemispheres and within those two hemispheres divided again by the different functions performed, I would have the brain set up in spherical layers similar to a jaw breaker. The first most layer would be the most crucial part of the brain and the outer most layer would be a part that isn't necessarily as important. Although all the layers of the brain are obviously important and have their own specific functions, the functions that they perform could be classified by level of importance. In the first layer, I would put the frontal lobe. Since the frontal lobe is responsible for speech, motor skills, the different emotions, reasoning, planning, thinking and problem solving it seems to be the most important part of the brain. The frontal lobe is also responsible for our personality, and voluntary muscle movement. Being in the front of the skull, the frontal lobe is susceptible to traumatic injury at any time as opposed to being well hidden and protected in the inner most layer. With the frontal lobe, I would somehow include the limbic system. Possibly putting it underneath the frontal lobe or even incorporating it in the same layer since the limbic system deals with emotions, memories and stimulation. Wrapped around the frontal lobe and limbic system would be the temporal lobe. Responsible for memory, speech and sound recognition I would personally classify the temporal lobe as the second most important part of the brain. The parietal lobe would be tied for second most important. Being that the parietal lobe is responsible functions similar to the temporal lobe, such as the perception of stimuli and recognition and also movement and orientation it's hard to decide which is more important. .
The occipital lobe would have to be the outer most layer.