@Azel re: Qyburn, the reason people are guessing Lich or something along those lines is pretty clear, A) Qyburn has shown no interest in planar body/immortality methods more conventional to the average sorcerer, like Lya and Zherys displayed in their own paths of study.
B) He isn't anywhere near adept as Anu in the making of very effective and yet high quality artificial bodies (the clockwork bodies for example that the Red Priests got with that ritual are for example not the most high quality or comfortable, lacking a number of features compared to Warforged who could actually drink potions and eat food even if it isn't required, I'm not 100% sure if they can taste them but it stands to reason that they
could since they can benefit from the magical effects of potions and Heroes Feast) so I do not foresee him purposefully choosing to walk down a path without consulting experts as opposed to working on it alone.
C) It can't be something related to dragons, even if that actually is a pretty straightforward research avenue to gain extreme longevity, and he seems like a man fond of elegant solutions rather than just putting something off for another few thousand years, and more importantly it would be an egregiously
bad idea to start down that path without consulting Viserys.
Therefore, given his skills and inclinations, it seems obvious it has something to do with either something Lich-like, or perhaps creating an undead body without any of the disadvantages, as it is mentioned that the lack of true sensation, the constant malaise of entropy hanging over them, is supposed to be inimical to coexistence with life in general. Vampires would be a great starting point as they get all the sensations without any of the baggage, just the dispensation of any and all humanity which Qyburn has very little of to begin with, just a will stronger than steel. But then he hasn't shown any hint of looking into that either.
I think of the three something along the lines of the third makes the most sense.