Batman's Superman
The superhero story to end them all: Batman is Superman. He's not Batman any more, oh no. He's Superman. For a bit, at least. Will he defeat the evil Zack Snyder, son of Ayn Rand? Yeah, sure. Then we'll make him (i.e. Batman) Batman again, and Superman can be Batman too for a bit.
Why? I'll tell you why. Why do you think? We know their mothers share the same first name but aren't related. To each other, that is. I mean obviously they're related to Batman and Superman. Not both of them (the mothers). Just one each. Batman's mum is related to Batman and so on. This begs the question - what happens if Superman is gone, and now Batman is Superman? What else, but the same brilliant story only this time Superman's not there and Batman isn't Batman, he's Superman. You see? The mothers names are still the same - it still works!