What should the scar look like after a punch biopsy?

Depends. Some surgeons, as myself, will suture the opening after a punch biopsy, so it looks like a line. Others leave it open, and it looks like a hole.
Depends. On some individuals there is no scar. On others a thick scar (keloid) may develop. On some patients there is a flat area of decreased pigmented skin. The size of the scar will depend upon the size of the punch biopsy. It has been my experience that how one scars in the past is representative of how you may scar in the future.