The game's definitely been a success. Though they've given no proper expansion for it they've still have been updating it with
faqs, new heroes and now physical cards.
The undead expansion was a leaked rumor that was looking like it'd be out by December but it's looking like it'll take my longer than that. The leak was probably from a old schedule that
GW had to change up, otherwise we'd already have a Tzeentch force.
I imagine, lore-wise, it's going to tie-in to the story hints of Nagash biding his time and getting ready for a big move.
[Edit]: Oh, welcome to the forum,
btw.