As they have almost everything in up-to-date sculpts and a functioning armybook, there is not much for a second wave left.
Oh, I'm sure they could replace those puff the magic dragon Cold Ones. But that wasn't my point - GW
have been hyping Fantasy armies with a new unit or two. And second wave parts are purely for completion sake. The idea is to get people to remember that hey, that army exists, then smack them with a book when their interest is up again.
Releasing the rumoured Carnosaur Cavalry a couple of months before the book would certainly scratch that itch.