Okay so here’s my theory, at first I thought that this was Slan before she was a Godhand but as many people have stated above, the curse mark located on her shoulder eliminates her from being a Godhand since she was intended as a sacrifice.
So who is this mystery woman who looks a lot like Griffith? Is this just some form of reincarnation? It would explain why my man Griffith is so damn sexy. However I don’t think that’s the case or at least the route that Miura isn’t going in.
So based off of the vision, we can conclude that this mystery woman is in the reason that the Skull King, the previous owner of the Berserker armor, lost control of the armor. It’s not revealed what the Skull King’s relation to this woman is but it’s obvious that it is some form of a love interest. SK could have lost control of the Berserker armor because of his love for this woman.
This is where I feel it ties in to what Guts is experiencing now. Gut’s hatred for Griffith is what allows the Berserker armor to take over Gut’s body. This is why I feel that Miura decided that the character design for the woman in the vision and Griffith look very similar.
I hate overanalyzing things and making too many assumptions but love and hate are two polar opposite emotions that are very strong. Skull King’s love for this woman and Guts’ hatred for Griffith were the things that consumed each user of the Berserker armor.
So in summary, this was an omen for Guts to be aware of his consuming hatred for Griffith because it will eventually lead to his demise.