Only the high rated celebrities with kids who like the show are most likely to be on it. Dan has some special connections, like somebody involved with "The Big Bang Theory" and Dr. Luke (hence Jim Parsons and Ke$ha). However, the KCAs and NIckelodeon appearances are usually by people with kids who love the show or people who aren't that famous.
Eh, I can see Katy Perry showing up though. Kids love her, Russel Brand was on BTR (who cares if they're divorced), she's sort of a kid at heart, she isn't really THAT famous, and she did the KCAs. I could see her doing something on Victorious, but I'm glad she hasn't showed up.