At the moment, it's looking like Daemons for 40K and Fantasy are next. Whilst the rumor mill has been saying we will be seeing this dual release around Feb/March for a while, I think I have been able to spot something to back this up.
If you read White Dwarf, you probably would have noticed in the back that the White Dwarf staff have been saying what they have been working on recently. If you have been reading this you may have spotted a trend.
For example, before the Chaos Space Marine release we saw updates from Andrew Kenrick saying he was painting up some Plague Marines and again before the Warriors of Chaos release, we saw him saying he was painting some Chaos Warriors up.
So knowing this, you may have noticed that Adam Troke has been painting up a lot of Plaguebearers for his 40K Daemon army and I think this is hinting that we will be seeing the Daemon release soon (within the next few months).