Callandor is vital for winning the Last Battle. This fact is made plain in Rand's conversations with Min in ToM. But what its role will be is still a big mystery.

It has been pretty much taken for granted since WH (where Cadsuane tells us about Callandor's 'flaws' and that it can only be used safely with a circle of 1 man, 2 women and led by one of the women) that it will be used in this way. The one man will be Rand, the Dragon Reborn and the two women two of his trusted companions (Nynaeve, Alivia, Cadsuane, Moiraine, his channeling wives... )

But this begs the question why it's the Dragon that's mentioned in all the prophecies and not women using Callandor? And from a thematic direction having two women in this circle doesn't fit this series. It's redundant, unbalanced and too 3rd Age-y by third.

And yet the universal acceptance of it should clue us that the Pattern pushes it as an answer or at least a partial answer.

There's also Min's doubts and uncertainty to consider. Clearly she's missing a piece of the puzzle. The plan as it stands has a missing element.

It's not hard to guess that the thing she doesn't know about is the True Power. Only Rand and the Forsaken know about it. And the True Power in the form of the Taint has shown a strong affinity to Callandor.

And adding the True Power to the powers of Saidin and Saidar gives us a 3 just like the supposed number of linked channelers.

Getting the True Power involved in Callandor's use, explains the repeated warning about the dangers of using Callandor much better than it's lack of buffer. This is the very essence of the DO we're talking about not just some mountain-creating uncontrolled power-spike.

The things such a circle will be capable of will surely be impossible for any other circle in the history of the WoT. We aready know that using just saidin and saidar won't be enough. The stories from the previous confrontation between light and shadow tell us this.

So if we assume that Callandor will be used in a circle with one saidin user, one saidar user and on true power user who will they be?

Saidar will obviously be used by Rand

saidin I believe will be used by Egwene. She has been paralleling Rand path throughout this story yet her accomplishments in the service of the light are negligible so far.

As for the True Power, the choice will be either Moridin, the big time user of the True Power throughout this series or Lanfear.

Most everyone believe her appearance in Rand's dream at the end of ToM is a ruse of some sort meant to trap and distract him, but I believe there's more to this. Lanfear is the perfect user of True Power for this circle. She has always lusted for ultimate, unlimited power. She is the only surviving member of the team that accessed the True Power in the first place. She is one of the forsaken, believed to be completely under the control of Moridin by the Shadow. And she is now bitter over the torture she has suffered under the Shadow's hand and eager to revenge if not atonement.

Rand will manage to free her from her current predicament and she will join him in a circle to defeat the Dark One and all his servants sacrificing herself in the process.