What if.....Peter Pettigrew was killed at the Shrieking Shack?

Sirius would've been proven innocent (if they probably would pardon Sirius for killing Peter....)

Harry would've went to live with Sirius.

Cedric could've lived.

Voldemort' rebirth would be postponed.

Buckbeak will die.

There is a chance that Ron wouldn't get Pig. (Unless, Sirius is still guilty)

Harry and Ron may not escape the Malfoy Manor (unless they have some other brilliant plan because the death eater will not hesitate)

Sirius will still die, but at least he got to spend more time with his godson and rushed to his death to save him.