An observant fellow on the BSN noted that each of the "statues" that mark the beginning of these DLC quests are placed on top of mats in Gamlen's house during Act 1. Two of those mats are now used up and two remain empty. Going by that, the popular theory is that there are at least two more pieces of DLC still to be released.
Based on some hints from the devs, everyone's best guess is that the next DLC will have something to do with a town called Wildervale.
All this is conjecture and speculation, however.