The only way to do a sequel now would be as a midquel before the last bit of 4, or to take over as Cassie. I doubt either of those are happening, because by the time Cassie takes over what else is left to find? And Nate/Elena clearly don't have any memorabilia of any extra events in that final scene. I guess you could argue since there's no/very little Golden Abyss memorabilia either that there may be adventures where Drake didn't find anything, but then that means no collectibles either, and if you're changing the core of Uncharted it's not worth it to even make the game.
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