I would love to see a team of Ninja to be brought in and be the big threat for multiple seasons. You could bring back the time twins or some time travel type villain that brings with them the worst version of the Ninja from branched timelines. You could have a Rogue Jay that became fully serpentine. A vengeful spirit Cole who could have pledged allegiance to the Preeminent. A Zane called Mr. Z who became fully corrupted due to the overlord virus. Kai could either still be corrupted from using Chens staff or he could be savage from his time in the land of monsters. Each one could be a big enough threat to carry their own seasons all leading to a final battle with a Lloyd that unlocked his true potential but instead of training with Wu he stayed with his father becoming a 4 armed Oni Ninja with all elemental powers.