Thanks to ikimashou.net, I was able to find this RPG Land interview with NIS producer Souhei Niikawa concerning Disgaea 2 and the future of the series:

RPGLand.com: “Let’s pretend for a minute that Disgaea 2 shows up in the US and sells awesome. Gimme the odds of a Disgaea 3, just from that. Would you specifically make a third game, or let it end at 2 and go the way of creating other original titles? Take a guess if you’re not sure.”

Souhei Niikawa: “Well sales number always helps for sequels, but we have no intent on ending this series just yet, so you can count on part 3.”