Please take this into consideration of Dreyfus' reappearance (SPOILERS)
I know they just couldn't do a Pink Panther movie without Dreyfus, and brought him back without explanation, and "Revenge" might be a prequel (and the last three aren't considered part of the Pink Panther universe by some fans), but this is what I think about the Dreyfus story. I think that Prof. Fassbender was always thinking of betraying his "boss," Dreyfus, because, even though Fassbender loves his daughter, Margo, he HATES world domination too. So, behind Dreyfus' back, he programmed the doomsday device to make things disappear, but then reappear much later, like in about 1 or 2 years. He must have known that someone was going to stop his "boss," so he wanted to save some innocent lives. He probably never expected it to be Clouseau himself. And thankfully, after the UN Building reappeared (and Dreyfus and Mondschein castle (the former then taken to the mental asylum)), Fassbender was probably cleared off all charges of unwillingly being part of Dreyfus' criminal mind and helping them "destroy" the UN Building. He must have known that Dreyfus was overconfident and was definitely going to fail at some point. So probably, while Fassbender was standing trial for unwillingly helping Dreyfus "destroy" the UN Building, its reappearance will clear his name.
So, I know you all will reject this, but this is what I honestly think. They may be crappy, but you can't deny the existence of the last three Pink Panther movies, and when Clouseau died after Curse, Dreyfus was still alive. And it's very obvious that Revenge is after Strikes Again. And I am aware of how they didn't think of my theory when they wanted Dreyfus back again without explanation.