Since it's the time of the year again, here come the challengers!
Subplots/Plot Twists:
Truth serum! At the worst possible time, MC becomes unable to lie (with optional inability to keep herself from talking).
Protagonist learns Antagonist is actually a decent person. Nothing else changes: Antagonist still wants to kill Protagonist, the two sides of the war are lined up against each other, Antagonist still killed Protagonist's pet, Protagonist is still holding Antagonist dangling over a cliff...
Your MC's mother shows up. Dun dun dun dunnnn~~!

The Butler actually did it. (OMGKUROSHITSUJI)
Does not legally exist (ie, no legal birth certificate, driver's license, social security number, etc., in any country)LOLS:MC discovers that his/her parents aren't actually his parents, and they took him in at a very young age because he'd floated into their yard in a balloon and nobody had any idea where it came from. Plus points if it's a silver weather balloon
'She looked like a moose. And not a very attractive one, either.'