Nippon Ichi announced a crossover data feature for Disgaea 4. Players can load their save file from Disgaea 3: Absence of Justice into the sequel.
Will this unlock a ultra powerful Prinny? Who knows. (Probably not, though!) Nippon Ichi is keeping the benefits of loading a Disgaea 3 save data a secret for now.
Speaking of Disgaea 3, the magichange system introduced in that game got a makeover in Disgaea 4. You can still change monsters into weapons, but that’s not all. This game has three kinds of combinations.
Human character + Monster = Magichange Weapon
Monster + Monster = Gigantic Monster
Gigantic Monster + Human Character = Gigantic Magichange Weapon