...I'm not really seeing how that's pointing to Demons in WH3 as opposed to DLC for 2.I'm not sure Nagash will be a playable faction at launch. The other thing that the second game hinted at was that the Vortex was being manipulated by some Slaaneshi demon, so I think it's quite possible we'll see at least one Demon faction playable from the start.
I can't really see them slotting in demons into the eye of vortex map, whose story more seems like the spark for a greater demon invasion than part of one. The ending of the campaign even uses demon invasion as a sort of punctuation, seen in how the Skaven are trying to use it.They're not making a game based entirely around Chaos, not if they want to sell at any rate. CA is well aware that it's "Heroic" factions (Empire, Dwarves, High Elves) that sell games. And never mind that there's no real story-hook for Chaos left - they already used Storm of Chaos and the Vortex.
Demons are probably going to be a DLC for Warhammer 2, what with all the hints (the vortex campaign, Bel'akor in Arkhan's epilogue, the Nurgle symbol in the Vampire Coast opening) and I'm sure they'll come with some new features for WoC, just like Vampire Coast brought bloodlines along for Vampire Counts.
Warhammer 3's going to be Kislev, Ogres, Chaos Dwarves and Undead, with the game's story being about the return of Nagash - Nagashizzar being the only big story-hook the Dark Lands have.