How would the meaning of the novel change if the narrator was just another unnamed human instead of Death?

Death represents humanity. He has no emotional attachment to the human world, and because of this, he is capable of giving the reader an objective, almost child-like view of humanity. Death spends a lot of time contemplating both the heroism and evil that people are capable of. His view is honest and straightfoward, unlike a human observer who would be tied up with preconceived notions and bias.