Before anyone can answer you need to refine your question ie change the C172...to make it better at what?
If hold 4 bums + lots of fuel + some bags then the answer is: Trade it in on a C182. But if the answer is to fly with 2 or 3 adults and only occasionally take 4, and long range isn't too important then the answer might be to do nothing.
I like the C172. I think it does a good job in its niche. Having said that, more nags under the cowling is usually appreciated until I have to buy fuel or maintain additional cylinders &/or turbos. Lots of avionic goodness is always appreciated - unless I have to pay for the installation, repair or subscription weather services.
The sorts of changes I'd make apply to damn near every aircraft I've flown:
* More storage space (racks/bins/compartments) for charts, approach plates & POH size books.
* Bigger vents and more of them so that lots of airflow can be brought in & directed where desired. Except for the BN2 Islander. Why the hell designers from a cool to cold country couldn't manage a steering linkage that didn't allow an icy blast through into the footwell is beyond me.
* Places to put pens/pencils
* Approach plate holder. Not just strong enough to clip on a page removed from Airads/Jepps but also able to hold a bound booklet *and* provide support so anything clipped there doesn't flop around. Also put in a position that doesn't obscure things and has a narrow spot of light directly onto it.
* 12v power outlets all seats, even in 24v aircraft
* Sunvisors that work no matter where the sun is.
* Decent control locks
* All cylinder engine data monitors (and balanced injectors, if appropriate)
* Certified to run on alternate fuels. I tend towards avtur because of its ubiquity.
* Standardised parts eg L&R tailplanes, elevators, ailerons & flaps and the like to be mirror images of each other so that repair & replacement is cheaper
* Centralised warning lights eg the way later PA31s have them all on the coaming instead of scattered all over the bloody place like earlier models.
Probably more if I thought about it longer but it's time for bed...