That's the kind of storytelling I love. Reveal something at the end that puts the whole story on its side. Something that gives you a reason to reevaluate everything you've seen, that adds layers and value to everything that has come before. Makes you question your assumptions and start deconstructing things in your mind. In short, stories that hit you with the pathos punch and make you think.
"No, I am your father."
"When I was picking coffee beans in South America .... "
"Is Ra's al-Ghul immortal? Are his ways supernatural?"
"Now that we know who you are, I know who I am. I'm not a mistake."
That's also what a lot of people around 1998 were hoping would take place in the prequels, specifically with the character of Darth Sidious. I avoided spoilers from 1998-2005, but I was going over some archives last week and it turns out there was this huge fan theory about Sidious and Palpatine, that one was the clone of the other. That made me think of Batman; how at least one real serial revelled in misleading audiences and playing with their expectations, and how Lucas's pretend serial simply pretended to do that through marketing and coy statements to fans.