Event Ninjas may come and go in events but they usually comes back when the Community wants them .. Oasis collects feedbacks thru their Event Feedbacks Forms where players says their thoughts about the current line up of events of the cycle or week, along with that, they also ask in their forms about what ninjas or items you want to see in that events, at the end if you want to say more you can suggest freely like suggest "i want this ninja to comeback in events to this events", and if the majority of the community says and suggested the same Ninja or Event, then the possibility of having them back is greater ..

No one can predict nor say that this or that ninja may comeback as soon as next cycle but the assurance of coming them back in one of the coming cycles is possible ..

Event Cycles is the event per week, that is what i meant about the word "Cycle" as you will notice at News Section, each week's set of events is called "Event Cycle" ..