It just occurred to me that I've never seen this done, so here it is. The Dumbledore SI. I had two thoughts for this one. Either the author could suddenly end up supplanting Dumbledore, gaining his memories strictly as information, starting at a spot throughout any of the first four books. It would be even better if this happened during an awkward time like in the middle of a big speech so the author has a challenge in dealing with a situation where they have to think quick on their feet, like perhaps appearing right as Dumbledore's about to speak the words 'Nitwit, blubber, oddment, tweak' and freezes as the author now has to figure out what's going on, deal with the situation he's now in, and other fun stuff.
The other thought I had, which may prove to be much more interesting, if far more challenging, would be waking up as Dumbledore when he's a kid, still best friends with Gellert, Ariana is still alive, and Abe doesn't yet hate your guts. The possibilities for this are huge, considering you can literally craft the wizarding world any way you like, or don't, since the main reason Dumbledore came to power in the first place was by dealing with Grindelwald, and the main reason Grindelwald pulled the crap he did was by taking Dumbledore's beliefs too far. Either way, it would be interesting to see what an author could do in Dumbledore's shoes, since so many agree that the steps he took in canon were just wrong. So here's the chance!
What would you do if you were Dumbledore?