User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 48: December 2015/@comment-27298431-20160107184628/@comment-1776824-20160107191820

I think that your scenario is overall likely, but it serves zero relevance to the forthcoming plot. And lol, Hiro is being very vague with "400 years ago", as the Dragon Civil War happened 400 years ago, but Zeref was an adult and immortal at that time, and he was a kid before that, and had spent a great number of years trying to revive Natsu, thus creating the other Etherious. I think Hiro needs to just be specific with his bullshit timeline, because there's no way this all happened within the same few years.

However, ultimately, Anna's existence was not emphasized like you say, but rather it is like he said. Anna was mentioned by Zeref because she opened the Eclipse Gate for her brother and the other Slayers, and then she had her clan pass down the knowledge on when to do so until Layla opened the doors on 7/7/X777. Her role stops there. There is no other feasible way she is relevant to the plot.