I've got to keep these handy. It's the idea that the character Leo Quintum in All-Star Superman is actually Lex Luthor, who has come back from the future after having seen the error of his ways as a result of seeing the world as Superman does.
Not-The-BeastMaster posted this one last year. Also, this one.
From the Savage Critics. Also relevant are the comments, as it's the first place I've seen theorizing that Quintum is Luthor. Let's see if I can find another...here...and here...we'll throw this one in there, too, though the part on Superman's a bit down the page...and here...and this one is relevant...and so's this, more or less...
I hadn't even thought of this, and after examining the evidence...I have no comment to make either way.