I figured out how to do it in an adventure.
The short form is, "What if the Cosmic Cube fell into a metahuman retirement home?"
The basic premise is that some wild reality-altering thing (possibly a reality-altering gem, maybe a mischevious imp like Bat-Mite or Mr. Mxyzptlk) comes into the Golden Age, a retirement home for metahumans. I had originally thought of a gem or cube, but I rather like the idea of it being an n-dimensional imp who has only their best interests at heart and a warped idea of morality. (We'll call him/her/it Imp Audible until a better name comes to mind: he's the genii who can only be heard. ))
The overt plot is tracking down a particular villain who has been running a group that steals, modifies, and sells high tech. This group has lots of mooks.
As the imp moves from resident to resident, the world of the heroes changes: for the ancient golden age hero, the world becomes as he or she remembers it.
The PCs are dealing with a bank robbery. Suddenly...
The PCs become Golden Age versions of themselves. The mooks become Nazis. Anything that happens during this "translation" is real--it's just the format, so to speak, that is different.
Everything goes back to normal, leaving the heroes confused. There is a clue that they decipher, giving a date two days hence and a place.
The next day, the hero's death is in the news. The Star-Spangled Sundog is dead.
It happens again, a day later. This time the Silver Age, the criminals become Communists. And the heroes are at a lab, preventing a theft of an item that is for sale. The buyer never showed.
Clever heroes will realize that the bank robbery was to pay for the tech. The communications the lab had with the buyer's organization gives them the clues they need.
The next day, word on the grapevine is that the silver age hero (call him or her Apocalypso) has died. The same clever heroes will suspect that the old heroes have something to do with the reality shifts.
Both heroes were at the Golden Age, a retirement and full-care facility for supers. If they investigate, the two heroes were both old and spent a lot of time playing cards with Melanoma, an Iron Age villain with Alzheimer's disease. It turned out they had a mutual interest in a three-handed card game of your choice. Melanoma's spouse is still alive, but had Melanoma was committed when he or she needed care, because Melanoma has a damage aura. (People who touch skin to skin get cancer of some kind. It used to be something that Melanoma could turn off, but now it's permanent.) The spouse hasn't divorced Melanoma (they're both in their fifties) but he or she finds it very difficult to visit. (The fact that Melanoma's spouse knows mostly supers is a way to introduce him or her back into another adventure.)
Melanoma was a criminal mastermind type. The ability is useful as a last-ditch thing, but it's not really useful against a master of fighting or someone who can take Melanoma down from a distance. (Powers: currently, Melanoma has the damage aura...in ICONS terms, it would probably be an incurable affliction...and the mental illness grants some degree of mental resistance. Physical stats are twos or threes. In Supers!, we're talking a Vampirism aura, maybe Persistent, and possibly a tied-in Physical Paralysis, and 1D or 2D resistances.)
But there's not really anything else unusual--the funeral for the Star-Spangled Sundog is set for today, so many of the staff are off-duty for the funeral--it's a sign of respect. One of the substitute nurses was on over the last couple of days, and she did hear voices from the respective rooms after lights out. That's not totally unusual, because sometimes criminals or dark avengers of the night visit after hours.
It's at the funeral service that some one mentions Imp Audible as one of Sundog's old foes, the one who challenged him so he could be better.
If the heroes don't go to the funeral, well, perhaps there are other clues.
The heroes are at the Golden Age because they've connected Imp Audible and the reality shifts, but getting the information from Melanoma is going to be difficult...and that's when the bad guys they've been dealing with all along attack. Because the heroes have interfered twice now and won't be allowed to interfere a third time.
And that's when it becomes an Iron Age world.
Imp Audible is there but only visible to special senses. The bad guys are there, armed with high-tech equipment. And he heroes are caught in the middle. Will they get help from the aged heroes and villains? Are there villains who prefer the Iron Age world of bleak moral relativity? If the heroes get to Imp Audible, he explains that he has given the Sundog and his friends one wish each, which lasts until they die or ask that it be stopped. Can they get Melanoma to ask that it be stopped? Because the Alzheimer's is bad, but Melanoma might last for another five years, though with diminished mental capacities.