GRRM's potential"big twist" in TWoW

I know this is old news now, but back in April GRRM said he was pondering a potential "big twist" regarding a major character in the series. Here's the quote in question:

In one intriguing new wrinkle, Martin says he just came up with a big, revealing twist on a long-time character that he never previously considered. “This is going to drive your readers crazy,” he teases, “but I love it. I’m still weighing whether to go that direction or not. It’s a great twist. It’s easy to do things that are shocking or unexpected, but they have to grow out of characters. They have to grow out of situations. Otherwise, it’s just being shocking for being shocking. But this is something that seems very organic and natural, and I could see how it would happen. And with the various three, four characters involved… it all makes sense. But it’s nothing I’ve ever thought of before. And it’s nothing they can do in the show, because the show has already—on this particular character—made a couple decisions that will preclude it, where in my case I have not made those decisions.” >

Source

I haven't kept on top of the various blogs to see if people have already spread around theories about what this could be or what character(s) it might involve, but I'm interested to see what people here think. The fact that it concerns a "long-time character" whose position in the show would preclude this particular change makes me think Sansa or Tyrion. There are other characters it could work with, too, but those two characters in particular are in dramatically different situations in the show than in the books.