Well what you can do is just about anything - what you need to answer is how much and how far do you want to go..
Theres nothing wrong with a clean tidy standard cinq tbh, that why we all love them
but if you do want to modify it a bit best place to start is suspension and brakes IMO, basic choices on brakes are either UnoTurbo front brakes or Punto GT front brakes. Punto brakes are probably better and are straight swap, just bolt the discs and calipers on and thats it, but you need 14" wheels to go over them. Worth noting there are 2 types, the later ones give better feel. Uno brakes are tiny bit more involved to swap but fit under 13" wheels. If you want to go further then a rear disc conversion - calipers can be taken from quite a few fiat cars (have a search if interested, i know there is a list somewhere on here of a load of cars that can be used as donor).
Suspension is a matter again of how far you want to go, lots available from just set of lowered springs right through to coilovers.
After you sorted turning and stopping then you can look at the engine. choices choices choices here. Mild tuning, induction kit, cams, exhausts, engine transplants, 1.2 8v's 1.2 16v's, 1.4 16v's, turbo conversions - anything is possible.
Styling wise - I like a subtle standard looking car myself but again anything is possible with skill and time OR money.
If that cinq was mine, i would get an abarth bodykit and spoiler, some anthracite sei abarth wheels and lower around 40mm all round. Perhaps sei abarth seats inside (the blue ones). But the important thing is what DO YOU want to do it??