I didn't like how the latest Ninjago seasons seemed to be going nowhere with the story, which might indicate that the theme would end in an unsatisfying way whenever it's time to pull the plug. If it were for me, I'd start building up towards the end in the next few seasons, and even if the theme continues to run for several years, we can still get a satisfying ending whenever the time comes.
As for how I would end the story, having the Ninja grow older and train a new generation of Ninja would be a perfect ending for me.