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(+1)

Okay I see, while I think it's a good think to keep the player in game after the end, I think it would be better to end here. Maybe, a screen with a narrative text telling how all this finally end, because we don't know what to do next, we wait for the next "objective".
But in the case you really want the player to explore more, I think the best thing would be to give more and more treasure maps, almost indefinitely.  Each time we find a treasure, there's another treasure map inside, to keep the challenge on.