Nentir Vale isn't new, but they may want to give us a break from too many settings too fast.
With the "publish and forget" model of settings, though, I don't feel that should be too big of a concern.
We've seen significant amounts of PS and SJ integrated into the "core setting" already, so who knows whether they'll get their own books eventually. I'd give that a strong possible, not necessarily probable though.
I do admit to being awfully curious, though!
My money is on no new setting, but merely a couple of gazetteers about different areas of the PoLands.
I wonder if there will be a new setting. The upcoming publication of Essentials and things like Heroes of Shadow suggests that they're changing up how they are doing things. There was an
interview or column a while back (can't find it for the life of me) where one of the designers noted that the one campaign per year release was something they were trying, but that they might revisit it in the future. If there was ever a year for evaluating the release schedule and doing something different, 2010/2011 seems to be it.
I could easily see them doing a bunch of little mini updates in the trade paperback format for certain classic campaign settings rather then one big campaign with separate hardcover books.
I think that aside from maybe Dragonlance they're hitting the point of diminishing returns for campaigns. Sure, a few people would be REALLY psyched about Mystara but the vast majority of gamers would shrug and ignore it.
Personally, I'd be very disappointed if that was the case. I have no interest in the PoL world.
What we want, and what we think might happen need not to be the same thing. That said, I do wish they would make PoLands into a real setting. But I am fairly easy to please, so unless they make Spelljammer or Dragonlance, I think I might be tempted to buy it.