Good question! (If I was Bill Condon, I would end it like this)
Breaking Dawn 1 Final scene: Jacob's face gasping as he just saw Nessie's human brown eyes.
Reasons: Bella is dead (technically no heart beat in this part of the book). Jacob is plotting the revenge/death of an infant (which he thinks is a MONSTER) As Jacob is walking down the stairs to the family room he no longer hears Bella's heart beat, he knows she's dead. And just as he's about to rip the child to bits, Rosalie turns around and he sees Nessie's eyes for the first time. Bam, he imprints. Which would explain why his cold callased face turns from plotting revenge/murder to instant shock/love/amazement. Plus, it would tie in with the New Moon ending (close up of Bella's face as she gasps after Edward asks her to marry him. Linking New Moon with Breaking Dawn would be an EXCELLENT idea since we all know it was anything but a good movie, but we love it anyways because it's part of the saga)
Breaking Dawn part 2: Starts with Bella screaming violently.
Reasons: (I would do camera tricks that show she's in an IMMENSE amount of pain) And the next scene be of the Cullens standing around her body congradulating themselfs on the use of Morphine to ward off the pain of the venom. Next shot (and camera tricks) to show Bella screaming (on the inside) and locking her jaw to stay quiet. Then the FIRST close up would be of Bella's red eyes opening (the screaming vs. the quiet atmosphere of the Cullens could all be down on camera within 1 minute) So basically the first major scene of Breaking Dawn 2 would be of Bella's red eyes opening...