Not sure if this could be done but maybe they could add items that would drop on loots or they could place on the ground. Something super powerful or extremely cool with an affect or a graphic. Similar to maybe a magic wand where you can equip the item and use its' power at the event. Maybe something with a timer rather than charges that would last 20 or 30 seconds. Once the timer runs out they could disappear. Something like a potion that you would drink and it would give you 1,000 hit points for 25 seconds. A weapon that anyone could use that would cause the spawn to catch on fire for 30 seconds or whatever was hit by the weapon. You could think of alot of things that could be used. So, an item that would give you like a super power for the event but would only last a short period of time so people couldn't use it any where but the event.