If Bella chose to abort Renesmee (or they otherwise didn’t meet), could Jacob have imprinted on someone else? Or was Renesmee the one and only person he could have imprinted on?
Imprinting, the Quileute believe, is a mechanism for helping to build a strong pack by leading them to choose mates who the best available, genetically for the most part, for the person doing the imprinting. As such, this would make it an entirely practical matter and, if Renesmee had not been born, Jacob would certainly have imprinted on someone else.
However, that would have been a wildly, radically different book.
Jacob imprinting on Renesmee is a key plot point, and there are several reasons for this. Most importantly, without Renesmee and Bella's pregnancy, the entire second half of Breaking Dawn changes. Bella probably goes to college, but in any case, Irina would not have seen Renesmee and incorrectly reported her as an "immortal child" to the Volturi.
Aborting Renesmee would have created a huge divide between Bella and Rosalie. Having a child was the greatest unfulfilled dream from her human past, and the main reason why she thought Bella's decision to become a vampire was wrong.
Now, that doesn't mean that the Volturi wouldn't come after the Cullens. In chapter 28 (The Future) of Breaking Dawn, there is a scene where the Cullens are discussing the coming of the Volturi.
"She decided to go to them," Alice said. "Irina decided to go to the Volturi. And then they will decide... It's as if they're waiting for her. Like their decision was already made, and just waiting on her."
The implication is that the Volturi wanted to destroy the Cullens, and were just waiting for an excuse that they could sell to the vampire community at large.
This then goes back to Jacob's imprinting on Renesmee. Because of the imprinting, the Quileute become firm, if reluctant allies of the Cullens. She becomes a glue that creates a large and more powerful family that eventually convinces the Volturi to back off, in the final confrontation. Without Renesmee, and with Bella and Edward off on their own, and a possible rift between Bella and Rosalie, it is entirely possible that when they Volturi found an excuse to go after the Cullens, that they might have actually succeeded in killing some or all of them. And without the Cullens, the Volturi might have gone after the Quileute separately, killing them as well.
This is just a sampling of some of the ways in which the story might have changed, as I said, it is impossible to know where the story might have actually gone.