The mysterious women be an ancestor of elena's family heritage. Perhaps Tatia isnt the original proginor of the doopleganger line. Wild assumption but its a really great possiblility, think about it everyone in this series is connected to one another in some way shape or form for example: Elena is the Katherine doopleganager who is currently dating and inlove with damon, that and dated stephen before she turned in to a vampire, Katherine is Tatia's doopleganger who not only dated both damon and stephan but responsible for turning them into Vampires But before meeting them she met the two original vampires Klaus and Elijah who was after her because of the curse of klaus not embracing his wolf half which made him a hybrid. Tatia who started the doopleganger bloodline was desired by klaus and elijah who fought over her and was deeply in love with her. In all of this the men that fell in love Tatia, Katerina (katherine), and elena were willing to risk their everything no matter the cost .
Season 4 :Silas creates the ultimate spell for immortality to be with women he loves and used his friend (Qetsiyah) to create the spell and the end he loses his soul mate, powers, and freedom.
Perhaps silas mystery lover could be a ancestor of elena!