Does it have to be just one thing?
Because my thing is perhaps a few things but only in one section of the car.
1. Possibly change the suspension design to the the US design IF it's better.
2. Replace the dampers and springs on the car with Koni FSD's and matched springs or Bilstein B14's. Perhaps offer both an call one the comfort pack and the other the handling pack.
3. Give the car a slightly sportier suspension geometry than standard. Halfway between standard and a race setup
4. Replace the rack ends, tie rod ends, suspension arms, droplinks and bump stops with better quality items or items which match the modified suspension
5. Find a good set of rubber which works for the car and the setup and only send the cars out of the factory with that so that there's a consistent level of grip available across the production run
6. Price it at 500-600 pounds
7. Watch magazines proclaim how great the 500 handles and how it's fun, chuckable and exploitable on the road.
Whilst I'm sure people will come up with some good ideas, I've never really seen anyone point out anything
wrong with the 500 other than things which are inherent like headroom in the back, but the suspension on the original cars was terrible and from my experience on a testdrive with a c, still wasn't exactly OMFGWTFBBQ good
EDIT: I'd probably also pair the above with 16" wheels and perhaps get a manufacturer like Speedline or OZ to make a set of nice light and strong wheels which could fit the 500 wheel centres and look "factory"