Ok I was thinking the same thing a while ago, but then I realized that all of the three classes basically have the abilities of an "engineer" class. And all of the abilities basically found in an basic engineer class are found within the guardians "drone" or ghost. You can do a variety of things with it, maybe even customize it's abilities we don't know yet but I don't see a need for an engineer class. Also the name "Phantom" wouldn't apply well to a engineer class, it is made more for the hunter type class, stealth and such.