Digging out an old issue of What If? last week really gave me a taste for the concept, enough so that you can expect some content on the subject. The original series was definitely my favorite, as mostly important moments and origins were used to spin off alternate histories. The second series was far less successful, wasting too much time on how recent crossover events might have turned out differently. As the book became more and more "90s", with gritted teeth, terrible art and Wolverine stories, it even lost its descriptive titles. You knew who was featured, but not what the What If was. It died the death it deserved, but it could have come sooner. What we might call the third series is basically a series of specials, usually based on a recent events, though there is some variety. I've yet to sample any of these, but I've heard that they can sometimes devolve into justifications for unpopular canonical choices.
It's possible that Marvel characters' origins and major historical moments have been exhausted, which is why there are so many What Ifs about the Age of Apocalypse or people joining SHIELD. It's too bad Marvel cornered the market on such stories with that wonderfully evocative title, because I'd LOVE to see a DC What If. There are a LOT of classic moments and origins to explore in the DCU. Elseworlds just aren't the same. Though a few of them HAVE taken the What If tack (like maybe Golden Age or that Superman Annual where the Kryptonians listened to Jor-El), most simply plop the hero into another time and space and retell the legend from that perspective. Even something like Red Son, which seems to ask "What if Superman landed in Russia as a baby?" isn't really a What If, not with the supporting cast and villains re-imagined to fit the Soviet world. (That said, What If also experimented with Elseworld-style stories at times, like having Captain America fight in the American civil war and Daredevil in feudal Japan.)
So for DC What Ifs, you have to look a little harder. That is to say, they're not all in the same place. They still have the largest backlog of "imaginary stories" going. Maybe if they had a Watcher to index the damn things...
What would be your favorite idea for DC What Ifs?
Off the top of my head, I'd try...
What if Supergirl had arrived on time?
What if Martha Wayne had survived?
What if Guy Gardner had been closer than Hal Jordan?
What if Plastic Man had remained a criminal?
What if the Invasion had succeeded?