1. & 2. - You have not seen the valuable cars yet then. Good luck actually getting your hands on the super cars like the Adder, Turismo, Entity, Cheetah, etc in the game world without having to buy it yourself. Def better than anything you'll see pedestrians driving around.
3. - Absolutely not. GTA has never introduced this "feature" and I can't think of a more lame, adrenaline-killing idea than to include a mini game where you must lock-pick a car in the middle of a 5 star police chase.
4. - You're not the only criminals in Los Santos, I've seen plenty of random instances where cops are not only patrolling but also chasing other criminals. Random shootouts. Random car chasing. All not including your character.
5. - Again just one more idea thats never been in a GTA game, and never should. Considering nearly every mission requires you to lose the cops before completed the objective or making it to a waypoint, I would absolutely hate it if I was about to end a mission but I forgot to stop at the stop sign, or ran a red light, or went 10 miles over the nonexistent speed limit. Not happening.
6. - There's plenty of side characters to meet and random events outside the story. Its a sandbox game, make your own fun. Go blockade the highway with some buses and pour gasoline all around one and watch the chain reaction of explosions occur. Get the big Chinook military chopper with the grappling hook and pick up cop cars and take them high into the sky and drop em, with the cops still inside the car. Try to tow a boat all the way up to the top of Mt Chiliad (damn near impossible).