|Cyndane Is Lanfear, Another
This theory never acquired a catchy portmanteau name - neither Landane nor Cynfear seem to roll off the tongue as easily as the ubiquitous 'Taimandred.' A shame, but it's not really necessary. We can all be fairly sure that Cyndane and Lanfear are one and the same person - in fact, in view of Winter's Heart we can be positive. How?
Cyndane addresses Graendal by name, makes snide comments about Moghedien, and generally shows herself to be unawed by the Forsaken. Only a suicidal Darkfriend, no matter how high, would behave in this manner. The only explanation is that Cyndane is one of the Forsaken herself. The only missing female Forsaken (since she channels saidar, theres no point looking among the men) is Lanfear.
Cyndane is in a similar position to Moghedien - subservient to Moridin, and most likely mindtrapped. Moghedien blundered and fell from her high position due to the desire to revenge herself on Nynaeve. Lanfear fell (literally) due to the desire to revenge herself on Rand and Aviendha, so logically suffered a similar fate.
Last Chance? If Lanfear is back, this is certainly her last chance to redeem herself in the eyes of the Dark One. Personally, I wouldnt give her this one. Furthermore, as Lanfear was the only one of the Forsaken originally to choose her own name - one which implies a high status in the Shadow - there's a nice irony in another name being forced on her that lets everyone know her current low status.
Cyndane is short, but stands tall, striving for every inch of height - as if she remembers being taller. Lanfear was tall. A minor point, but the kind of hint Jordan might drop.
Cyndanes arrogance is very similar to Lanfears customary behaviour, and Cyndane is as scornful of Graendal's collection of pets as Lanfear had been previously.
Finally, Cyndane's point of view in Winter's Heart shows her thinking of Rand as Lews Therin (as few save Lanfear do) and that:
She would have faced the Great Lord - faced the Creator! - with him. She would have shared the power with him, let him rule the world at her side. And he had spurned her love, spurned her!
No one except Lanfear has reason to have such thoughts. But she certainly does, since shortly after dropping her Selene disguise she told Rand:
"Two great sa'angreal were made just before the end, one that you can use, one that I can. Far greater than that sword. Their power is beyond imagining. With those, we could challenge even...the Great Lord himself. Even the Creator!"
This scene makes Cyndane's identity clear; the only point against it is that, according to Demandred as informed by Mesaana, Cyndane is weaker than Lanfear had been (although still extremely strong, as we saw from Graendal's point of view scene in The Path of Daggers). This problem is solved (although the solution raises yet more questions) by another sentence from the Cyndane scene:
She (Alivia) was stronger than Cyndane had been before the Aelfinn and the Eelfinn held her!
So, Cyndane has encountered the Aelfinn and Eelfinn (yet more proof for Lanfear, if any more was needed) and in doing so somehow lost some of her strength. Puzzling. The only way we know of to reduce strength is severing followed by imperfect Healing. Yet according to Jordan, the effects of severing do not last through reincarnation; after dying and being placed in a new body, Cyndane/Lanfear should be back to full strength. Even if reincarnation by the Dark One works differently to normal rebirth, it leaves the question of who Healed her.
An alternative - we know very little about the Aelfinn and Eelfinn to say whether it is possible or not - is that she made the traditional three wishes while within their domain and paid with some of her strength. The price may also have included her life, thus freeing her for reincarnation, or the new body may be the (perhaps unintentional) result of a wish.
A persistent theory is that Cyndane is indeed Lanfear, but in Moiraine's body, merged in the same way as Luc/Isam. The Eelfinn may indeed have this power, as Slayer is known to be associated with the Tower of Ghenjei. There are - possibly - similarities between Cyndane and Moiraine, but there are also differences. In particular, Moiraine has dark hair and eyes, whereas Cyndane has silver hair and blue eyes.
The similarities: They are both short. (Counter: So are a lot of people.) Graendal describes Cyndane as 'a beautiful doll carved from ice' while Cadsuane in New Spring calls Moiraine 'a pretty little doll of a Cairhienin noble.' (Counter: Liandrin is also described as doll-like, and for sure she isn't Cyndane.) When asked if there was a resemblance between the pair, Jordan responded not with a simple no but with "We don't know yet." (Counter: Jordan loves to taunt.)
According to the FAQ, it has also been suggested that Cyndane's original body is that of Cabriana Mecandes, the Aes Sedai interrogated by Semirhage in Lord of Chaos. Both have blue eyes and long hair; silver hair and pale hair could also be considered to match. Possible, but I think unlikely: I'm guessing all the bodies of the recycled Forsaken came from people as obscure as possible, to minimise the chances of being recognised. On the other hand, for obscure Borderlanders, Cyndane, Moridin and Aran'gar all seem to have rather striking looks.
Although Taim=Demandred is now in doubt with the revelations of Winter's Heart, Cyndane=Lanfear is not merely confirmed - but irrefutable.