Longevity is a key indicator of a classic design, and going by this standard the bicycle must be just about perfect. While many have tried, the basic layout hasn't changed since the Starley Rover safety bicycle of 1885.
The latest attempt to meddle with the classic layout is this monowheel designed by Ben Wilson for the XXIst Century Man exhibition at 21_21 Design Sight in Tokyo. Even Wilson himself is ambivalent about his creation, saying “We don’t suggest for one moment that a functioning human powered monowheel could ever provide an improvement on the modern bicycle. Instead its value comes from the discourse and ideas generated as a result of this research.”
I guess Starley got it right.