I've only read through a few pages on this question. Some have voiced concerns about continueing the romance aspect and some have said everyone has a happy ending. There are other directions thing could go without the romance aspect. But for romance there is Leah because she does not yet have her happy ending. I'm interested in her story, how she learns or if she does learn anger management, go to college, she might have to wait for one of the younger boys to grow up to imprint or wait longer for one of current ones to have boy children. Or maybe finds love without imprinting.
Also for romance, Jacob and Nessie. I think she will grow up thinking of Jake as a friend / best uncle.. When she looks 18yo, her actual age will be 7-8yo. As Nessie is growing up, she won't be able to have the social experiences outside of family. Ok, she learns and matures rapidly, but there is still life experiences she will need to go through to be ready for a mature romance with Jake. He will have to wait for her and I think he will have to court her before she sees him in a romantic way. With a mind reading father that could be interesting to say the least.
The Volutari conflict is the non-romance way to go (though Jacob/Nessie could be a part of that story). They have to come after the Cullens side again and it will likely by a physical fight(s). They took a hit and won't be able to just let it go. I imagine Aro setting up some of the talented guard males to make some hybrids as a way to get around Alice as well as to see what ways the talent might permutate. They might have to find and change a good doctor / researcher depending on what find from Nahuel's father or maybe not change him, just get him to work for them.
There are some prequels that could be interesting. We don't know alot about Alice's searching years. Even some stories of Carlisle as traveled and learned through Europe. What historical things might he have personally witnessed? More background on the Denali clan, including newest member Garrett. Even Victoria might be interesting to see how she came to be a vampire. James not so much, I don't think he was a nice human. Victoria, I'm a little iffy about her as a human.
There is always fan fiction to go in what ever direction with which ever character most want to know more of.