What about like week long events?
Genin vs Jounin: based off the OVA
Basically Genin drop Blue Crystals, which are worth 1 point and Chunnin+ drop Red Crystals, which are worth 3. The objective is to collect as many Crystals as you can by the end date to recieve rewards, which have a 1st-3rd place ranking. And then maybe a consolation prize for the top 50?
To avoid abuse by spam killing newbs, you can have it so you can only drop a Crystal if you currently hold any.
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Dungeons:
In this event a Dungeon will open up for an hour, in this hour you will be able to kill a mob(mob depends on which dungeon is opened) and collect summoning pieces, collect a certain amount and you will be able to summon them as a pet.
(thought about this and then saw the Toad Summon one. Excited about that!!! but wanted to post this anyway)
Great Ninja War:
All players team up against one common enemy. This NPC(Masked Man?) will have insane stats and will take a combined effort to kill him. While he doesn't regenerate Stamina over time, he does gain 10% of the max stamina of those he kills.
Shogi: Kill the King:
An NPC will be summoned in each village which will not do anything. The purpose for the village is to Protect their King while also trying to kill the other villages King. This will pit Leaf vs Suna for a while since we will only have those two, but once other villages are added, it will be an all out war. Before the event starts, using your village pool, your Kage can higher mercenary ninja to help out your village to ensure victory.