I have one suggestion to make both developers and players happy. I don't want this as a Halloween event. Instead, it can be introduced as "Horror Tunnel" in the city itself like placed next to friends tavern like mountain tunnel entering.
To keep this as a limited time event, this completely bad idea as per my opinion. Because there are easy failures and profits to players which are not that much compromising ones. They can tweak the teeth concept easily and can be introduced as separate concept in the city. They can improvise so much in that way by introducing in the city area. All players can wish to do daily 3 dungeons and reset behaviour fits nicely to consider as a new concept in the city. League reward, prizes just need to be replaced or altered, thats not a big deal.
Also, bring the last year Halloween event back with new changes and kindly requesting to consider this thought for a moment to make players and everyone happy. It's a win-win situation for everyone. I strongly feel like that and mainly too bad to throw this new event as one time activity in the entire year. Always, GE, GvG, GbG, PvP, Plunder, GB contributing, goods trade, let them have this as a one extra content to the city. I don't see what's wrong in doing like this.