This isn't the "best" possible ending I can think of, but it is definitely an ending I can see coming (it's just a prediction so I could be WAY off....)
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Knowing that by turning Bella, a war with the werewolf pack could ensue, Edward risks it and bites her. Just as the pack is preparing to wage war with Sam in the lead, the Volturi appear. This complicates matters and the pack and the Cullens are forced into a temporary truce once again. Jacob has long returned, BTW.

During the battle with the Volturi, Leah is killed trying to protect Sam, but Sam gets hurt anyway. In his state of vulnerability, the Volturi attack him a second time (since he's the leader) and kills him. Jacob is forced to take the Alpha male position to fight w/ his brothers and to keep Bella safe, even though he knows she is now his natural enemy.

After a victory against the Volturi (the surviving vampires return to Voltera), the pack are eager to fight against the Cullens. However, Jacob - out of his love for Bella - orders the pack to stand down. He tells them that they are never to attack the Cullens (especially Bella). The pack, although angry, cannot go against the orders of the Alpha male.

As his final act of love for Bella, Jacob tells Edward to take her away - somewhere safe where he (Edward) can look after her until she becomes a "vegetarian". Although Jacob wants to say one last good-bye to the love of his life, he can't bring himself to face a vastly changed Bella who, as a Newborn, could also very well attack him (since she is not going to be herself for a long while). He says a sorrowful good-bye from afar, and watches Edward, Bella, and the Cullen clan leave Forks for good.
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It's kind of a sad ending for Jacob, but in a way, it makes him an even stronger (and appealing, I might add) person. Even though I wish it, I just don't see SM introducing a new person for Jacob to imprint on - not this late in the series anyway. Perhaps in another spin-off series...?

Does anyone have predictions??