Nothing!
To say that the C72 is a mature design would be a severe understatement.
I'm pretty sure that Cessna has improved everything which could be improved without having to design and certify a new airframe, or add an unacceptable amount of empty weight to the airplane as it now exists.
The 172 does a good job of keeping those without a great deal of experience, like me, out of trouble.
It is a durable and forgiving machine, and has had a good record of getting the SOBs back to earth without failure in bad conditions.
The 172 is not a fill the tanks and fill the seats airplane, but it was also designed in an era when the typical American was smaller and lighter.
If you want an airplane that can usually carry four pax and full fuel, Cessna would be very happy to sell you a 182.
If you need yet more useful load, there is always the 206.
Both are more expensive airplanes that will use more fuel and cost more to maintain.