Interesting idea. However, I would change a lot of things.
I will forward this suggestion to the rest of our team and we will see what they think about this.
While I like this idea, I can tell you rewards are going to be different (if this will ever happen). Maybe it can be event about fragments of ninjas but definitely not new ninjas. You know, at the moment there is nothing like EDO sound four or Cursed seal version of them. So first they would have to make them, animate all of their skills and many more things.
Also, in the future we will have more events so I don't think it is good to add even more events (like Ninetails, Group convoy) because there is really high possibility that players will miss some of them.