Okay, assume hypothetically that Marvel or DC was going to go out of business personally, no one wanted to buy the rights to their characters or properties, so they decided to have one huge event to conclude their continuity, with everything coming to a finale and loose ends being tied up, etc.
In other words, this would be the last story of the Marvel or DC characters, that would end everything.
So how do you imagine it would be?
You can write what you think it would be like for Marvel, DC, or both
The problem is that DC and Marvel are both fictional Universes based around an open ended Meta story. There's no way you could properly "tie off" either of them in one story without making it a crapfest, even more of a crapfest than most of their current events.
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“Where the longleaf pines are whispering
to him who loved them so.
Where the faint murmurs now dwindling
echo o’er tide and shore."
-A Grave Epitaph in Santa Rosa County, Florida; I wish I could remember the man's name.
Cosmic war between Death/Oblivion and Eternity/Infinity, that goes out of control of the Living Tribunal. Thanos is behind this shit, and Death/Oblivion want everybody dead.
The last issue would be this: Death and Oblivion finally killed everyone, but Galactus. They fought him, beat him, and tought that he was dead, but the mofo's still rising. Still, his energie's reserves are almost depleted. He uses what's life, to gives him enough energy to use his tech, to absorb Oblivion, and use this energy to outpower/absorb Death. Galactus is floating in the nothingness, alone. The Pheonix Force come to him, they both talk, and then Galactus makes the ultimate sacrefice: Pheonix Force ignite him, he explose, and a new universe is born.
Superman wakes up one morning to find Lois gone. With super-vision he looks across the city, then the entire planet, and finds nobody else is left on earth. He flies into the sky but sees everything fading away. He hears a voice saying "Yes Superman, it was all a dream. Marvel and DC, these universes would never have existed if not for you. But you yourself are just a dream, a figment of the imagination. And all dreams must end one day. Your time has come."
Superman screams as the Marvel and DC universes fade into darkness and oblivion.
Then the reader throws the comic into the bin and says "well that sucked. I'm going to go play X-Box."
Spider-Man lost his cat and do a deal with Mephisto because he wants him back. Mephisto agreed to giving back, but Spider-Man most gives something: His past memorires, since the 1960s. Spider-Man really likes his cat, and sign.
The Brothers (from the mid-90s crossover) find themselves spontaneously re-retconned and clash again, directly, this time seemingly obliterating both the DC and Marvel universes.
Once the blinding light disappears, we find Jerry Siegel and Joe Schuster awakening in 1932 from an alcohol-driven dream-stupor. They look at each other, go "Naahh," and return to working on "Reign of the Supermen," chuckling at the fading wisps of what they think they dreamed.
Siegel stops for a moment. "Hey, how about a schizo hero, y'know, someone with a dark, evil side...
Schuster: Just stop, Jerry. Just stop. "I tell ya: Reign of the Supermen" is gonna rake in big bucks..."
[camera pulls back, we see a dark-haired, suited narrator with a lit cigarette. He begins...]
"Submitted for your approval: two talented young men, caught in the free fall of the Great Depression, lose themselves in their imaginations, never realizing they themselves are reprints ... in The Twilight Zone."
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Shinier than a speeding bullet.
I think the most "reach to the reader" way to end them would be to have the characters realize they are fictional, or at the very least, they are always watched by someone who thinks of them like heros and the comic is just a heart to heart from the characters to the reader. Could be well done if writen right, and while ABSOLUTELY cheesy, I think it conveys the propose of super hero comics in the metatextual way that Morrison would do if he was told to finish comics
All the heroes vs all the villains in one massive free for all that literally tears your comic in half. For DC it would have to come down to Superman vs Lex Luthor or rather Clark vs Lex. It's always been down to that. For Marvel it's got to be the Father of the MU, Reed Richards vs it's ultimate villain: Dr Doom. It should be completely resolute and stress once and for all that dead is dead.