Technically Rockstar wouldn't even need to alter much. Throw in a few meters tracking hunger, body heat and thirst, toss in a crafting system, remove all the NPCs, hide supplies around the map and for the love of god add animals into the multiplayer version of GTA: Survival.
As an aside, it wouldn't surprise us to see such a mod for Red Dead Redemption 2 either, just look at the popularity of Undead Nightmare.
Narrative Content For GTA Online
We recently wrote up a long piece advocating the addition of story DLC to GTA Online as opposed to the game's single player component. Players have been clamoring for Story DLC for three years now, and initially it was Rockstar who hinted that it's coming. Since then, no news has surfaced about it, other than a number of fake leaks.