Before we get to the new cameos take a look at a new job, Disgaea 4 has an Android class.
Cast Omega Wind and you’ll see Holo from Spice and Wolf.
Index from A Certain Magical Index is the guest character for Omega Heal.
Disgaea 4 also has a "guest" from Disgaea: Hour of Darkness. Pre-order the game to add Flonne (in HD!) to your party.