I would end it with you being alive (obviously) with all of your friend dead. and there are always going to be insane people on the island, so that you could still be in free roam and just do little missions, like how red dead redemption was.
and at the same time, also leading into a beginning of farcry 4.
Would be fun to be able to finish the side quests after having done the main story. TBH the first thing I thought about when I read the title was that the game ended with the player unlocking something for the map editor or multiplayer.