Sadly, I don't think a Brad Pitt rendition of "Twist And Shout" would be nearly as charming.
This theory is the essence of what makes fan theories fun. The plot holes (Why are they taking attendance in class for a kid who doesn't exist?) will always be there, but the theory brings some of the movie's themes and subtext into the foreground, giving us a closer look at why Cameron finds Ferris so appealing. Cameron probably didn't imagine Ferris, but if Cameron did have an imaginary friend, that imaginary friend would be Ferris.
Why It Doesn't Work Everywhere Else:
This is probably the internet's favorite fan theory. The only ingredients needed are one character to be imaginary, and another character to imagine them. It's hard to find a movie, TV show, or video game that doesn't meet those qualifications. You only need two characters. The activities on the back of a box of Franken Berry provide all the evidence you need in order to theorize that Boo Berry is a manifestation of the immortal Count Chocula's survivor's guilt.
One iteration of this trope has it that Steve Buscemi's Donny from The Big Lebowski was really a ghost that fought alongside John Goodman's character, Walter, in Vietnam.
If this theory contributed anything worthwhile or showed us a different way of looking at Walter, maybe we'd be willing to try and overlook the fact that multiple people interact with Donny in the movie. Unlike Cameron, however, Walter is far from self-loathing. Unlike the narrator from Fight Club, he is anything but repressed. The biggest problem with the "Tyler Durden" is the fact that it's often shoved into a story with little thought as to the circumstances. Before you can add in a proper Tyler Durden, the character has to need a Tyler Durden.