So this idea was born when OUAT said that David has a twin. So why didn't Emma have a twin sister or brother? Emma seems to take on the fighter side of her parents, rather than handle being Queen. Caroline Forbes, a fighter and capable of handling royal duties.
Okay, so what I have thought of so far is that Caroline and Emma were separated for a little while but then grew up in Mystic Falls. When the girls turn 17, Emma is given a chance to go to a special school for criminal justice or maybe she graduates early and goes to college. Emma meets Neal, falls head first, doesn't know he's a thief. He gets caught and tries to drag her down with him, but she is able to walk away.
Meanwhile, Salvatore brothers are in Mystic Falls. Damon seduces and compels Caroline. Just forewarning, Damon is a bad guy at first. He does rape and abuse Caroline. Elena and Bonnie call Emma, telling her about the situation with Damon. Cannon only with Emma staying around to help, also Emma's pregnant. So when Caroline becomes a vampire, Emma is brought in on the secret.
Up to you on how to handle the next few seasons of Vampire Diaries, but until it reaches Once Upon A Time. Maybe Emma did give Henry up for adoption, maybe there was a summer camp in Storybrooke. Up to you. While Emma is skeptical, Caroline believes. Determined to prove it, she calls Bonnie. Bonnie says that there's stagnant dark magic, holding things in a stasis, like a plastic bag.
Now for the portal to Enchanted Forest, David, Caroline and Emma would be the ideal ones to go. With Caroline going up the beanstalk with Hook if you ship them.
I seriously ship Emma and The Huntsman, but you can pair him with Bonnie, Caroline, or if you really feel it vital; you can keep to canon and let him die. But another reason I also wanted to do this story was to see Killian Jones with Caroline. Please, please let this happen!