if you could change one thing about btc, what would that be?
i think like many others i wish bitcoin had faster transaction times. but wishing is not changing. because changing things like the time between creation of blocks and many other things would mean sacrificing a lot of other things.
Would you make it safer?
bitcoin is as safe as it gets. Cold Storage means maximum security you can ever hope for.
How would you develop the btc trading?
trading has nothing to do with bitcoin so it is a little strange asking it among these questions.
the change that trading requires is becoming "decentralized"