I really don't hope jumping from rooftop to rooftop is going to be Crackdown-style, or something similar. That wouldn't be GTA. Flying with motorbikes without falling, bunny-hopping 3 meters or getting shot a thousand times without dying doesn't annoy me
But flying while being on foot does.
And about the trunks, I really hope it'll be included. Like Himuro said (I also said this in the wishlist), being able to get rid of witnesses, or anything you want to get rid of, put it in your trunk, drive to the water and dump it.
Being able to keep weapons in your trunk would be great, too.