I think it's pretty obvious that DC's current title, Tales from the Dark Multiverse, is kind of like their version of What If? However, it does seem to be a version of What If? with a specific bent to it: the twist to the old story is intended to result in the worst possible outcome. In the Knightfall and Death of Superman one-shots, the only two released so far, Bruce and Lois basically become murderous psychopaths...in different ways, mind you, but still. And I guess it makes sense, since its supposed to be the Dark Multiverse.
It got me thinking, though: what would it look like if Marvel did something similar? What would be some story arcs that Marvel could revisit and introduce a plot divergence that would cause a "Dark Multiverse" style reality?