Are you not understanding the destruction of the mood?! It would occur because the game is meant to be serious, a panorama of death and destruction that you are desperately trying to circumvent. Having cat ears wouldn't fit because it would become a distraction from the more serious themes.
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I fail to see how it could ruin the mood at all.
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How would it be distracting at all? I play PSO2, where characters have multi-colored wings, cat ears, giant swords, massive bows, strange hats, Rappy costumes, and I saw someone yesterday whose character was dressed exactly like Hatsune Miku. None of that took away from the story of the game at all, or the serious quests.