22. Here (2014)
This one is a bit tricky, in that Richard McGuire originally debuted the concept behind Here as a six-page comic book story in an issue of Raw in 1989. Decades later, he took that idea and then made it as a graphic novel. I don't think that that disqualifies this book from being considered an original graphic novel, though. It's the CONCEPT that is being used again, not the actual original comic book pages.
Anyhow, the idea of the comic is a breathtaking idea - just draw one single spot, throughout time itself. Pick a corner of a room and just depict that same spot at different points in time. Sometime there was no house there. Sometimes there were people in the room, other times no one. McGuire also mixes in the time periods so that you have a scene from 1970 in one part of the room and 1940 in another part of the room. Stuff like that.
That's a really clever idea, right?
21. Gotham By Gaslight (1989)
This is the graphic novel that launched the concept of Elseworlds at DC Comics. Written by Brian Augustyn with art by Mike Mignola and P. Craig Russell, it takes the idea of Batman and his various cast members and translates them to the time period of the late 19th Century.
As you might imagine, this sort of thing is right in the middle of Mignola's wheelhouse and he depicts this Victorian era Batman design beautifully...
Of course, this being 1889, you can't NOT work Jack the Ripper into the story and Augustyn is no fool. He has the Ripper come to Gotham and the people of Gotham believe that Batman must be the Ripper. It's a ripping yarn that inspired a whole imprint at DC Comics!