Typical lurker thing, pop in for one theory loaded post and never write anything again... I would prefer to post this in some other thread but it seems to overlap with several different threads and I can't decide which would be most appropriate, so I will leave that to mods if they think it the right way. Parts of this have been suggested at or said by others...
Basically train of thought starts with Hannah. The implication that she's a dreamer, emphasized by Zoe's reaction rubs wrong and I go with the shifter idea, thinking nighttime shifts are dreams. Combined with the drawings it puts her as a solid reborn April/Saga candidate (and maybe not even nighttime shifts yet but more past life regression stuff... or what is it, anamnesis...?).
I also think Anna as part of the chain with all the similarities others have noted. She's obviously older, and others have written that off as some dream form or whatever but I'm suspecting some House of All World's time-foolery. So it goes that a young Hannah is embittered and confused and after whatever discovery she undergoes in the future of her heritage, past-life(ves), and powers decides to right some wrongs through the HoW wormhole and goes back to "an important time" to work with Kian. She just gives off that whole "back from the future, knowing you already though you don't know me yet" vibe.
This is all a bit crazy but if the April-Saga-Hannah-Anna chain ends in Lady Alvane, it's not the first time that Lady Alvane has gone back in time through the HoW to correct some details from one of her stories (TLJ as we all remember). Going further, this implies to me the theory of Lady Alvane being the White Lady from Dreamfall, "rebooting" April to bring about her Anna incarnation to move things along a different path... redrafting the story as it were. The Alvane chain also goes back to the oft-quoted idea of Ragnar's: to create a game where you play through the same character at different life stages. It's just clever of him, if correct, to put them all acting to a degree within the same general time frame.
Changing gears but ultimately related I think, the first Dreamer and Zoe's shared uniqueness and importance (even Draic-Kin haven't seen Dreamers before) make me think of that dreamer of the world, Vishnu. He had many different avatars who were born and lived within the world as incarnations of himself to right wrongs being done (Rama and Krishna being famous examples). Reminded of Zoe's dreaming herself places and dreaming things real, I wonder if she isn't herself a dreaming form, or incarnation, of the sleeping First Dreamer... The sleeping Dreamer is dreaming itself into the world as Zoe, that is, to combat the Undreaming.
From that perspective, Lady Alvane takes on a different interpretation to me. She becomes less older April/Saga whatever and more a complimentary figure to the Dreamer, the Storyteller perhaps. Dreams, the child, provide the raw material and shape the world and from this raw material stories, picture and art are fashioned. A wise, old woman sitting beside a fire in a house outside of time telling stories in opposition to a young child dreaming the perpetual dream of the world. Of course this starts to overlap a little with the Vagabond in function but from his own words he tends the individual threads and can't see the larger weave which maybe goes more to Lady Alvane... If correct this could suggest the importance of her avatars to be "waves" and create individual stories within the larger Dream, to "fix" things. Etta and the importance we sensed in her stories also coming to mind here.
This starts to suggest a twist on the Trimurti (even the Moirai a little...), which leaves the Shiva/destructive force in the hands of the Undreaming? the Thief of Dreams? If all of our characters are avatars of these Trimurti type forces, Kian? Did anyone else find the Crow(?) cameo in Book 2 slightly creepy and put in mind a certain "Is that you funny bird?" moment? Or maybe the Vagabond is the third (Thief of Dreams an interloper on the balance of the trio) and the roles are a little different (i.e. Dreamer/creator, Vagabond/maintaining the fire, Storyteller/bringing stories to a close...)?
I think that's it out of me...