As I said in an earlier post, Smallville should end with a nod back to the Season 6 episode Labrynth. The entire series should be Clark in that mental institution along with John Jones as a fellow inmate, and that he has imagined everything based on his own delusions which have drawn from Chloe, his sole visitor, having been reading stories to him from this comic named Smallville. Parts of Labrynth would've been the only true episode of reality because in that one Clark would've been temporarily coming out of his delusion, but slipped back into it since as believing the fantasy was more hopeful for him than believing the reality of being a mental patient.
At least a twist ending like this would have people talking about it for years to come.