Marvel's characters and series seemed well-suited to its WHAT IF--? concept with plenty of good alternate concepts to postulate.  In fact, many of them later actually happened!  With long arcs like (I'm paraphrasing) "What if Jane Foster were Thor?", "What if the Hulk had Bruce Banner's Brain?", "What if Conan Walked the Earth Today?", etc.!

DC, on the other hand, gave us dozens and dozens of imaginary stories and ELSEWORLDS.  Some of these could easily be phrased as What Ifs--"What if Superman and Lois got Married?" (plenty of Silver Age imaginary stories could have been called that) or "What if Baby Kal-El's Rocket Landed in Russia?" (RED SON) or Britain (I did enjoy TRUE BRIT!) or today or the Old West, etc.  GOTHAM BY GASLIGHT certainly sounds better than "What if Batman was in the Victorian Era?"  Like that one, a lot of the ELSEWORLDS simply placed people's favorite characters in different time periods.

Besides these few examples, can we come up with good WHAT IF ideas for DC characters?

Remember, Mort Weisinger was the king of "A Hoax! A Dream! An Imaginary Story!" It seemed that a quarter of his office's output was Superman marrying Lois.

Julie Schwartz and other editors weren't quite so into such tales, but to be fair, Wonder Girl evolved from a What If story...

For new concepts, I'd like to have seen, "What If Bruce Wayne and Dick Grayson were the same age?" Joe Chill and Boss Zucco's men strike on the same night. At the churches, two young men, angry, bitter, and grief stricken encounter each other and vow that no one else will ever suffer as they have. One night, a bat crashes through a window into the home they share, and one sees a bat, and one sees a dark wing - but both see an omen, since criminals are a superstitious cowardly lot.

What would you like to see?

Or: WHAT IF Jeanette Kahn and Dick Giordano at least had an actual plan in place for what the DCU would be like after the end of CRISIS ON INFINITE EARTHS and therefore none of the continuity "snarls" caused by the reboots of Superman, Wonder Woman, and the HAWKWORLD monthly series had ever happened? 

Or: WHAT IF DC stopped trying to force characters like the Marvel Family into the same world as the JLA, Teen Titans, Doom Patrol, etc., where they clearly don't belong? 

Or: WHAT IF The Dark Knight Returns had never been published and the "grim and gritty" take on Batman (among others) had never happened?

I could go on and on...

"Hey, where did the Batcave go? Why is my costume red? Won't I get recognised with just this domino mask?"

"What if Batman had stayed married to Talia?" 

"What if Lana Lang had moved to Metropolis with Clark?"

In  a way maybe it's interesting to imagine stories like this-

"What if Wally West was the only person with speed force?"
"What if Hal Jordan (or Guy Gardner or Jon Stewart) were the only Green Lantern"

I agree with what someone said, DC changes so much so often that a What If story wouldn't mean much. I do have one though, What If the Silver Age DC started with the Flash and all their characters that were being published before his debut didn't exist on Earth One?

...Barry Allen got splashed with different combinations of chemicals and became a different super powered individual? The Atom? Aquaman? The Wildcat? Spider-Man*?

...Sinestro were given Abin Sur's mission instead?

...Plastic Man could change into different chemical compounds instead of stretch?

...Superman had a permanent super powered sidekick, i.e., Skyboy, the Boy Marvel?

...Tritonis and Poseidonis went to war with each other, and that battle spilled over onto dry land?

...Katar and Shayera Hol were in fact not only Thanagarian law enforcement officials, but advanced scouts for an invasion of Earth?

...the Legion of Super-Heroes never went back to the 20th century to actually bring their originating inspirational heroes into the future?

...the Legion actually had been based in the 21st century instead of the 30th century?

...Supergirl dated and eventually married Robin the Boy Wonder?**

...Gil Kane had gotten the "other" Superman gig instead of Al Plastino?

...Bruce Wayne, heartbroken after the death of his parents, tried to retrieve their souls... and became the Spectre?

*Remember the timing on this event. Showcase #4 easily predated Amazing Fantasy #15.

**Yeah, we all know who Robin is; but I love to type his sobriquet!

I hope these are sufficiently intriguing. I got more... :D

What If Joe Chill Killed Bruce Wayne instead of Thomas and Martha? (yeah, I know they've done something like this with Earth 2. But there's loads of scope with this as a divergence).

I'd like to suggest a Superman story, but his continuity is so buggered up, I don't know what's considered canon, so I don't know what to diverge from! And there've been so many "Imaginary stories" with him. But I like glimpses of what he might've been like had he been adopted by another couple rather than the Kents. 

Edited to add: Wee edit done, as my knowledge of Earth 2 was erroneous! :)

Wonder Woman- What if Enemy Ace crash landed on Paradise Island?