It looks like you don't have to worry about Darksiders II and the ability to play it entirely on the GamePad, as developers have informed Polygon that the game is compatible with said feature.
Evidently this feature was left out of a few preview builds, but now we know the whole story. In addition to the included Argul's Tomb DLC, the follow-up DLC packs are heading to Nintendo's Wii U online platform for sale, so no worries there either!
Honestly, I hope this feature is available for pretty much every game on the system -- Nintendo's huge selling point is that the GamePad is its own television, so hopefully it has more support.
Darksiders 2 supports GamePad-only play [Polygon]