I agree with most of the opinions I've read.
So, silver or pewter hardware
more comfortable handles (in order to wear the bag also on the shoulder easily)
slightly longer strap for the city and the first.
Resolving the yellowing/fading issues
An easy care service, in order to send the bag to Balenciaga and having it back like new, with a honest fee.
Oh, yes, RT every year would be a dream!