COIE/Post Crisis DCU fix:
I'd open with a series of scenes of the darkest moments in DC over the last 30 years or so, the mass murders of Crisis, the Post-Crisis murder "Superman"'s execution of the PZ villains, the butcherings of Zero Hour, Doomsday, Identity Crisis, Infinite Crisis, Flashpoint, etc...then I would have the Earth-One Superman wake up from the nightmare, his last adventure being a JLA case from JLA 182 vs Felix Faust.
At the next JLA meeting in the satellite, Superman would mention these nightmares, and he would find that all the other JLA members had them as well...Flash has dreams of him dying over and over, Batman had dreams of his back being shattered, Joker killing Robin, paralyzing Batgirl, etc...eventually they would contact the JSA, the Legion, The Marvel Family, the former Titans members, etc, leading to a big cosmic series involving Spectre, Phantom Stranger, Shazam, and other forces that would reveal that the Multiverse heroes were having visions from is a "Pocket Multiverse" that was created by the Time Trapper, a duplicate of the True Multiverse with the exception being that there was but one Oa in this Multiverse (COIE #8 stated that there was but one Oa, the Oa of the Earth-One universe, but prior stories with doppelgangers of Hal Jordan and slight variants of Earth-One contradict this. Eventually all of the heroes from the classic Multiverse would meet their Post-Crisis counterparts, and the real threat would be discovered: that by manipulating Krona into creating the Pocket Multiverse and by causing the Anti-Monitor to exist, Time Trapper had caused the Pocket Multiverse to be unstable, ever changing, never being settled, and it would eventually destroy itself-and take the True Multiverse with it. In the True Multiverse, the Earths and Universes had developed naturally, but in the Pocket Multiverse, Krona's interference at the beginning of time had caused the Earths to split. A team of the most brilliant scientists including Brainiac 5, Terry Sloane (of Earth-Two), Michael Holt (of the Post Crisis Earth), Alexei and Lex Luthor, etc, invent a counteragent that can be injected into the matter at the beginning of time in the Pocket Multiverse that will allow those Earths to duplicate themselves in a more stable manner, keeping them from changing and allowing them all to exist. So the Post-Crisis Earth would be one Earth- the Post-Zero Hour Earth would be another, etc, up to and including the Post-Flashpoint DCnU.
The end result would be a DC Multiverse where every character and every story in canonical somewhere, somehow. I'd give the Pre-Crisis characters a few books, including allowing Roy Thomas to finish All-Star Squadron on Earth-Two and I'd make sure the Earth-One Superman had a book. I'd also make the Legion book continued from the Pre-Crisis history and have them as the Legion of Earth-One. I'd keep the Lee/Johns Justice League series as it is and I'd have a JLA book on Earth-One starting with number 262. It would mostly be the classic JLA with Wally replacing Barry, who would go to live in the future with Iris after Flash #350.
I'd also have a JSA series sat on Earth-Two in the present in All-Star Comics starting with issue 75.
The rest of the line would be the Post-Flashpoint DCnU series, but any series from any Earth would certainly be a possibility.
Oh, and Sensation Comics would be brought back, most likely on the DCnU Earth, as a Wonder Woman team-up title.
