Lateral thinking
Like the previous posters, I agree that the C172 is pretty good at being a C172, so if you think on conventional lines, you will just end up designing the C182.
But, I would rush out and buy a C172+ instantly if it had (1) no extra payload at all, (2) went 10% slower, (3) cost just as much money to buy, but (4) was as quiet inside as a typical modern small car.