You know what I'd really like to see? Trickery. To add on to my previous ideas (of various "trick" effects that could be applied upon saying "trick or treat" to a specific NPC), I would like to see Halloween trickery in sorts of forms throughout the days leading up to Halloween.
I'm talking about things like baddies in Graal City, a temporary new loot item that's some kind of hand that pulls you underwater and warps you to some kind of underground pool of water, having bugs occasionally "break free" from your net and turn into some kind of zombies that attack you. "Fake" gralats that are actually baddies that will attack you upon being uncovered, evil versions of known NPCs taking the place of the real NPCs, that infamous "big hand" enemy from Zelda occasionally coming down to pick you up in the underground railroad area and warping you to the nearest exit. How about shield codes being totally randomized one day? Where you put in a shield code and end up with a completely random one. The same can be done with swords. And maybe your head/hat/body randomly change every 30 minutes or so? (And you'd have no control over any of it). Everyone would look crazy and have no idea what's going on.
These are the types of things I would LOVE to see. Just screwing with the players in honour of the occasion. I want the devs and the admins and everyone to just unleash their fury. Just one week of total chaos. Man, that would be fun.