Wow.. It doesn't feel like 4 years since the HE got an update, still would have thought they'd be a long time off.
At the moment, no one really knows who the next army is. There is very strong speculation that in a couple months, GW will be coming out with a WHFB equivelant of the Space Hulk box. Currently it is expected to be a 10mm scale naval game. If it proves to be true, there will not be a new WHFB army release until next year.
You'd have to expect either WE, Brets or Dwarfs will be next since they are the oldest armies. However, all of them do function in the current rules, don't expect a GW update just because they are old. GW will update the oldest/weakest model line first, and probably the army that has the most metal units still in them. They will also look for one that they can introduce new units and a big kit or two easily, since that is what is selling for them. I'm not sure which army that would be off the top of my head.