DISCLAIMER:Any theory that is posted is just in fun. I am posting these as a reader myself, imagining I'm in your shoes and what you would think. None of these theories have potential spoilers in them and can be taken how you want to take them. They may or may not be true, but that's up to you to decide.
These two people - The Green women and the Raven have had a BIG appearance throughout the books, but they've simply not done much stuff. They have a hand in a lot of stuff but seem to be chilling. Who are they?
Well let's go through their appearance chronologically. The raven appears in Gerold's first chapter, telling him to fly and Gerold has all these visions, the green women appear and tell Gerold that he must go up north. The common motif with them is that Gerold must go north and north and north. In Gerold IV he recieves visions again, the green women tell him that he is their only hope. Aeyron also receives visions from the green women, but instead he sees his ancestor marrying them and the women tells Aeyron his family have also had a relationship with them.
What can we gain from book one then? Both Gerold and Aeyron recieve visions from the green women, Gerold recieves it for the future while Aeyron recieves visions of the past. These green women have mythical powers then that means they can merit visions in some people; they tell Aeyron that the Sun's Champion and The Winter's Son must meet, i think it indicates both Gerold and Aeyron to meet. But what are the green women? They've existed for years before seeing as Aeyron's ancestors used to marry them, but what are they now? We get a lot of information about them in the second book.
In the second book, Aeyron stops recieving visions and to an extent Gerold does too - Gerold is now in communciation with them. Gerold stops dreaming about the women following WinterFort's burning, but dreams about them where he envisions Allerax Blackfyre's tomb. At the tomb, there are caves there and a giant Mythood tree superseding it. Gra remarks that he felt a connection to the place due to the Mythood tree. Let's analyse the Mythood tree right now. Lord Cregan Forrester explains to Jon Darklyn that the Mythood Trees are all-seeing, and that The Old Gods watch through it. Whenever Gerold dreams about the green women it is because he is in the prescence of a Mythood tree, he dreams at WinterFort, he dreams of Allerax who's tomb has a Mythood tree next to it. Gerold doesn't get dreams of them in The Winter Waste seeing as it's full of mountains and there are no Mythood trees around. It is possible then that the green women keep watch of Gerold through the Mythood trees like how Lord Forrester suggests that the Old Gods watch through them. This suggests that the green women are Old Gods then. However it may not be the Green WOmen that are giving the vision and rather the Mythood tree itself, Ser Jamie Casterly sleeps on the stump of a 'strange, white and ancient tree' and he gets visions too; although his are a mixture of the past, present and the future - unlike Aeyron and Gerold's. Perhaps they are not the Old Gods then. Another thing that links the green women to the Mythood trees is the wolf that joins Gerold's side, its eyes are red just like the Mythood tree leaves. The green women possibly sent it, and Gerold believes so too ('the wolf you see with me. They sent it'), the way they know about it and how they tell Gerold it will direct him.
Seeing how the green women had a relationship with House Archer, it wouldn't be a surprise to suggest that they existed at the same time as the First Men. We know that at the same time of the First Men lived giants and the Dryads of The Forest. Maester Colon suggests that there was 'something magical behind the Dryads', Samuel also tells Colon that he's seen the Dryads and that they're very much alive. In book one we are also told that 'the Dryads of the Forests worshipped the Mythood trees', similiar to our green women who use the Mythood trees to see things right?
But now we have the Raven to question. He seems his own thing, totally unrelated to the green women but he exists alongside them. The Northerners consider the Mythood Trees as a part of the Old Gods, that the Old Gods watch through the Mythood trees. Perhaps then the Raven is an Old God, or maybe there are no Old Gods and are only one God that is the Raven.