Mary Margaret using Regina's teardrop in "Second Star to the Right" is a hint to their relationship getting to an even higher level. With Zelena's curse altering reality, the only way for Regina to be safe (since the Wicked Witch's curse would make her have never been born) would be merging with her former archenemy. In a way similar to Neal-Mr.Gold's body-sharing, Regina will be one with Mary Margaret. Snow White and the Evil Queen will become literally Snow Queen (Elsa). As to Emma, her name in the original script for the show was supposed to be Anna.