There have been many imaginative Harry Potter fan theories that seemed like great ideas - until Book 7 came along and crushed them into the dust.
Some look totally deranged in hindsight - Harry and Hermione were siblings? Get outta here! - but there's one theory that was surprisingly popular before all of the books were published:
Professor McGonagall is a traitor to Dumbledore.
This is how the argument went, as discussed by HP fan K.S....
McGonagall often acts mysteriously...
At the very start of Book 1, McGonagall in her cat form is surprised to see Dumbledore at Privet Drive. Dumbledore says, "Fancy seeing you here, Professor McGonagall," suggesting he didn't ask her to meet him there. Why is she there?
Did she form an early bond with Voldemort?
McGonagall was born in 1925, Tom Riddle in 1926. That puts them at Hogwarts together at the same time, where they would have interacted, especially as concurrent prefects.
She puts Harry in danger
McGonagall makes Harry a Seeker, putting him onto the broom that Quirrel jinxes. Remember, this is a witch so determined to follow rules that she denies permission for Hogsmeade visits without a permission slip - but puts him in the way of aerial accidents, even providing him with a broomstick.
McGonagall's contribution to the Stone
Remember when Harry, Ron and Hermione approach the Stone at the end of Book 1? They trash the chess set she'd transfigured, which leaves the question: how did Quirrel get past it without breaking anything? Maybe he had some help...
McGonagall: Voldemort's "most faithful servant"
In The Goblet of Fire, many fans speculated about who Voldemort meant when he referred to his "most faithful servant" being at Hogwarts. Some thought it was too easy that this meant Barty Crouch Jr., and that JKR was storing up a big sucker punch in the form of McGonagall being revealed as a traitor...
McGonagall tends to wear green (unless she wears tartan) - a small sign of allegiance to Voldemort? She also refers to him occasionally as 'The Dark Lord' - something only his followers do, generally. Also, JKR often hides character hints in their names: The Queen of Spades is Minerva - McGonagall's first name - and Spades mean trouble and death.
Personally, I think that this would have been a great, massive reveal - but was never very likely. What do you think?