What if Superman had revealed Supergirl's existence immediately upon her arrival?

What if the Kryptonians escaped destruction in the Phantom Zone?

What if Superboy never caused Lex Luthor to lose his hair?

What if Batman's parents had lived?  (Alan Brennert did one version.)

What if Batman had a brother?  (Bob Haney referenced this, but no one else did.)

What if Bruce Wayne fought crime before the bat crashed through his window?

What if Batman prevented the Flying Graysons' deaths?

What if Earth 1 Superman were never Superboy?

What if Earth 2 Superman DID start as Superboy?

What if the Martian Manhunter were the first super-hero of Earth 1?

What if there were Earth 1 versions of Dr. Fate, Dr. Mid-Nite, Hourman, Sandman, Starman, Spectre, and Wildcat (or did Mike Fleisher and Bob Haney do those last two)?

What if the Challengers of the Unknown gained super-powers?

What if the Creeper, Metamorpho, and Zatanna were villains?  (Marvel did a similar issue.)  --or the Metal Men?  --or Hawkman & Hawkgirl?

What if the Teen Titans grew up to take over as the Justice League?

What if Abin Sur didn't die?

What if Barry Allen were struck by a lightning bolt and DIFFERENT chemicals?

What if Oliver Queen didn't get stuck on that island?

What if Supergirl joined the Teen Titans?

What if Superman lost his powers to Gold Kryptonite?

and

You Erics are so different in the way you think :)

What if there were Earth 1 versions of Dr. Fate, Dr. Mid-Nite, Hourman, Sandman, Starman, Spectre, and Wildcat (or did Mike Fleisher and Bob Haney do those last two)?

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I think the First Issue Special issue with Dr. Fate was meant to take place on Earth 1. And, there were characters called Sandman and Starman on Earth 1 as well. Just very different from the Earth-2 versions much as the two Atoms are very different from each other compared to the other counterparts.

Eric J. (see?) - "What if there were Earth 1 versions of Dr. Fate, Dr. Mid-Nite, Hourman, Sandman, Starman, Spectre, and Wildcat (or did Mike Fleisher and Bob Haney do those last two)?"

I felt that Dr. Fate and the Spectre were so powerful that their areas of responsibility included multiple Earths. That probably included Earth-3 as well, although DC Comics Presents #29 shows us what would have happened with Ultraman taking on the Spectre - nothing good, that's for sure.

Earth-1... no Dr. Mid-Nite. No Hourman. Sandman was the entity with Glob and Brute as I recall, but that all got confused up depending on who was writing it, and what week. Starman was both Prince Gavyn and earlier (sic) Thom Kallor. Per Bob Haney, Wildcat and Sergeant Rock and Blackhawk inhabited both Earth-1 and Earth-2. Professor Hugo Strange inhabited two Earths, as well as Batwoman, Robin, and Ace the Bat-Hound.

But Brave and the Bold and (especially distressing) the New Adventures of Superboy showed that there were heroes who pre-dated Superman's appearance on Earth-1. Air Wave, Sunfire, and Zatara were fighting crime while Kal-El was growing up. COULD Crimson Avenger and Doll Man have been out there? Sure. Those two are purportedly the first super heroes (on Earth-2) but Kent Nelson was studying magic at Nabu's side before the Man of Steel made his first appearance. Ditto for Khufu and Shayera.

Those appearances get a little quirky... heroes in the past always did. (For example, Super-Chief preceded Superboy's first appearance, to be sure.) Makes it so hard for us fans who simply dig too hard and too deep into DC history.

Except for maybe Gavyn, none of these were really INTRODUCED as Earth 1 versions.  And no second version of Spectre, Wildcat, Sgt. Rock, or Blackhawk appeared (and got zapped) in CRISIS, so I believe the thinking was these characters were on "Earth-B"--Earth Bob Haney or Murray Boltinoff where (according to Bob Rozakis in letters pages) out-of-continuity Earth 1 stories occurred.

Same with Jack Kirby and the Guardian and the Newsboy Legion. Also with Dennis O'Neil and Air Wave. And Archie Goodwin and Manhunter (Paul Kirk). And whoever it was that brought back Zatara in the '60s (Gardner Fox?) as well as Sargon the Sorcerer (Lein Wein?).

I'm not even sure which Earth that Commander Steel -- a '70s character living in the '40s -- was originally supposed to be on. 

Pretty sure he could remember that Superman and Batman never married anyone and had children...and then all of a sudden there are the teenage Super-Sons in every other issue of WORLD'S FINEST COMICS!

With all his writing of JUSTICE LEAGUE OF AMERICA (including JSA team-ups) and also having GL Alan Scott dimension hop to team up with GL Hal Jordan in GREEN LANTERN, I'm going to go ahead and say that Denny O'Neil was fully aware who was Earth 2 and who was Earth 1--he probably just thought it would be a fun thing to connect Larry Jordan/Air Wave to Hal Jordan/Green Lantern.  (Making the Golden Age Air Wave Earth1's first super-hero?  If you can call roller-skating along power lines a super-power!)

Little did he realize that later writers would connect EVERYBODY with similar last names!