I think the Parasyte tentacles would be an interesting touch as new cosmetics on Lunar Client. Similarly to the anime and the series, there can be multiple parasyte cosmetics with different tentacles patterns (some with blades, for example) and making them move a bit when the player is walking or sprinting would also be a good touch. In addition to that, I think it could enrich this idea if the tentacles would sync with the hits (making an attack gesture to the opponent you are hitting to make them seem realistic).
In case the attack gesture is not viewed as a good idea, the parasyte tentacles could also be a head cosmetic with no gestures and just a head thing. To conclude, I think this would be a nice cosmetic to be created.