I have no idea as to what they are going to do plane-wise. I'm assuming they won't return to a plane that they've announced before, so probably not Vryn or Chandra's home plane.
As far as story line goes though, I'm pretty sure there might be an Emrakul centered block coming next. I think in Eldritch Moon they will figure out that it is Emrakul, she will get pulled in close enough to cause some problems but never really arrive because Sorin will kill or beat the crap out of Nahiri, then Emrakul will escape to a nearby plane and actually show up this time and start effing stuff up. Then in the next block I think the Gatewatch or another group of planeswalkers will have to fight off MEGA EMRAKUL!!!!!!!
I have no idea if that's what it is going to be like, but I am pretty confident that Emrakul will be heavily involved.