Alan Moore, right before he decided to never work for DC again, pitched what has been the Holy Grail of DC Crossovers for a few decades now: The Twilight of the Superheroes. It was a full proposal which DC bought, and then has essentially been sitting on ever since, unwilling to try it without Moore.
Hints of it turn up now and again. In fact, it’s fairly safe to say that Kingdom Come was heavily inspired by said proposal, although it’s pretty different in the details and doesn’t take the original concept as far as Moore did.
But, as Rich Johnston points out, recent events in DC continuity are starting to ring a few bells.
… we have Superman himself, a morally troubled figure who doesn’t know what’s best to do about the chaos he sees surrounding him, but who has come to accept that the Houses provide the only real permanent structure in a destabilizing world and are thus important to maintain.
That pretty aptly sums up the current Supes, to some degree (one of the flaws in Twilight, which Moore acknowledges, is that Superman in current DC continuty would never actually accept this).
Also, he’s married to Wonder Woman. Shazam, meanwhile:
… Captain Marvel himself… is if possible even more estranged and troubled by the state of the world than Superman is
There are other little touches, like Constantine essentially being married, that stand out. It’s a bit thin, but then again, DC is showing an increasing willingness to use Moore’s material without Moore being involved.
It’s more likely that we’ll just see parts of this used in DC’s upcoming Trinity War, but still… it’s intriguing.