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It would be better to put horde mode in, together with overrun and survival, rather than leave it out, because it will be able to appeal more people with different preferences. Horde mode is always a plus, even games like GTA Online and other new games added it because horde is good enough to be a standalone game. Just like mercenaries mode in Resident Evil can be a game on its own, Resident Evil Mercenaries the game.