There would have been a campaign for each character:
-Trevor (Agent DLC) would have continued on doing missions for the government (this got recycled into The Doomsday Heist, people on GTA Forums
found references to shit in the source code like Clifford and the jetpack years ago), continuing the story from the main game
-Franklin (Alien DLC) would have fought off an alien invasion (the ones from Michael's drug trips; that saucer from Mount Chiliad are remnants of this) in a what-if scenario with an altered map
-Michael (Zombie DLC) and his family would have fought in a zombie apocalypse a la The Walking Dead (this would have been next-gen only because the more powerful hardware would have allowed them to render more of the motherfuckers than previously allowable) in a what-if scenario with an altered map
-The casino was a single player DLC heist (they found the casino references too) as part of the Agent DLC
-That mystery mural? Was alluding to the DLCs (zombie egg, ufo, jetpack)
Sources:
Rockstar Games was once planning to support Grand Theft Auto V with single-player story DLC, like [...]
comicbook.com
https://twitter.com/TezFunz2/status/1142452560420376582 (this one says it was almost
2/3 completed)
At least one original DLC story for GTA V was scrapped, if data miners are to be believed.
www.dexerto.com
What happened to the story expansion we were promised on the games initial release? Its been 5 or 6 years and theres been no word since Online became the only way to get all the DLC, which I dont even have home internet so I basically own 20% of the game. Hopefully they will finish the story and ...
gtaforums.com
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gtaforums.com
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I have all these links because I was interested in the subject and looked into it at one point, even posting it in chat.