The idea calls for a lobby where both the main map and minimap is disabled, players are given the weakest pistol with low ammo and the objective is to kill everyone else. One life, no respawns. There would be no PEDs or NPC cars driving around the map. Weapons, vehicles and consumables would be extremely scarce and hidden in strategic locations, with the Military Base housing the best gear, but it would be defended by NPC soldiers.
Of course, to make something like this work in GTA Online, quite a bit of fine-tuning would be required. Many players have suggested additional mechanics to prevent matches turning into several-hour long games of boring cat-and-mouse. After a set amount of time, a sudden-death zone would appear on the outskirts of the map and slowly decrease in size, forcing players close together, similar to "Penned In".
Another possible solution to this would be starting off with a smaller area from the get go instead of letting the players traipse across the whole map, and impose a health decay feature that can only be halted by scoring a kill. Another way to prevent matches from dragging on too long would be a periodic "sonar blip" of sorts being released every 5-10 minutes to show where the other players are.
Either way, this would be a pretty time consuming game-mode, no matter how Rockstar mixes and matches things. The payouts should be adjusted accordingly, making it a higher reward activity due to the higher investment of time required from players. Seeing as GTA Online's Adversary Modes have been plagued by low payouts, one of the main causes of their low number of players, this mode would have to avoid that route in order to ensure a decent amount of people playing it.
While this game mode would be pretty far from being grounded in reality, Adversary Modes have always been pretty crazy and out there. Even if this Battle Royale game type wouldn't act as an Adversary Mode, with the recent Cunning Stunts update taking a leap into fantasy-land, anything could happen.
Would you enjoy such a gamemode in GTA Online?