I certainly think we will have some more kits coming with the book release. Cavalry will be one unit, although what cavalry I can be certain. It looks like there are three subfactions:
Beast-breakaz: Dealing mainly with beasts and monsters
Deffspikerz: The shooting element
Badstabbaz: The bog standard troops
I guess there might be a way to have a battleline unit for each of those subfactions. Gutrippaz fit into the Badstabbaz nicely, Boltboyz fit into that for the Deffspikerz and we just need to see a character that allows them to be battleline. Finally are the Beast-breakaz. I can imagine we will see some cavalry and they could be battleline for this subfaction. I'm not sure if that's Hobgrotz or Orruks but I certainly think we will see some cavalry. The Breaka-boss can then make them battleline for the subfaction.