The possible consequences for each season.
- Day of the Departed: Would still happen, but would be so much more depressing because Nya is now dead and Jay and Kai would have to mourn her.
- Hands of Time: Would still happen, but Nya would not be there to retrieve the Reversal Blade with Kai, and so the Time Twins would never succeed in their plan, the Iron Doom would never become operational, and Wu and Ray would end up dying because Acronix time-punched them into the future. Since Nadakhan twisted Lloyd’s wish, Lloyd would have been really old at the end of Skybound, and would also presumably die at the end of Hands of Time without the story ever revealing he was part Oni and dragon.
- Sons of Garmadon: Lloyd is now dead due to unnatural old age, and so Harumi’s years of planning the SoG would now go to waste because she has no one to get revenge upon anymore. She would then just be an ordinary person, with no means or motivation to resurrect Garmadon. (remember, Lloyd is not around for her to collect his hair.)
- Hunted: Would not happen at all. Garmadon is not resurrected, so he never creates the Oni Titan, which never crushes the Bounty, which never leads to the ninja using Traveler’s Tea, which never leads to them being marooned in the realm of Oni and Dragon.
- March of the Oni: Would not happen at all. Aboard the Misfortune’s Keep, Flintlocke accidentally destroys the Realm Crystal in an attempt to shoot Cole for intruding aboard the ship to steal the map to Tiger Widow Island. It is revealed at the beginning of S10 that the Oni enter Ninjago through the Realm Crystal.
- Secrets of the Forbidden Spinjitzu: Not quite sure about this one, but it may or may not happen. At this point, only the original four would still be alive. They would presumably be in a very VERY rough state, lacking the desire to find adventure, so there is a chance they would never stumble upon Aspheera’s pyramid. But if they do, Zane would not become the Ice Emperor because he would not take the shot for Wu (he dies at the end of Hands of Time, remember?), and the whole battle sequence in the monastery would not happen because Aspheera would not know where to find Wu, the treacherous deceiver, and therefore never proceed to attack the monastery.
- Prime Empire: Will happen as normal, just that Nya would not be there to accompany Jay through the Temple of Madness and he would have to do it alone. The dancing episode would not exist though, but the four ninja still alive will find another way to get the 500 credits they need to obtain a Key-Tana from the Speedway Five Billion.
- Master of the Mountain: Since Harumi was never evil in the first place, she would not die in Hunted since it never happens, and then she would never be resurrected by the Overlord and ordered to buy Vengestone, so Vangelis would not have needed to enslave the Geckles and Munce, and Cole would have never fought with him. The ninja would have left after Vania’s birthday party and proceeded with business as usual.
- The Island: Would happen as normal, except Lloyd would not be the mascot character, and the Storm Amulet would not hold any relevance.
- Seabound: Would not happen at all. Nya would never be captured by Kalmaar while trying to discover what was causing her powers to go haywire (she’s dead). Kalmaar would never find out the location of the Storm Amulet because Nya would never accidentally reveal it, and so Wojira will still be asleep and the Great Flood would never happen.
- Crystallized: Depending on whether or not S11 happens, Aspheera would either still be trapped in her tomb, or trapped in Kryptarium. The ninja would never break her out because Nya’s been dead for nine seasons now and therefore never merged with the sea. Harumi would never form the C.O.t.C.K because she was never evil in the first place, and the Overlord would not be manifested into the Crystal King because the C.O.t.C.K would not help him obtain the Golden Weapons, so therefore the season never happens.
So to show all the haters who don’t like Skybound’s ending, Ninjago would have turned out vastly different and infinitely worse had Jay not reversed time. Not to mention Nadakhan would still be on the loose and be an ever present threat, and Djinjago would still be a thing. This just goes to show that Nadakhan caused arguably the most damage out of any villain thus far, and it took an impossible act to stop him. Aside from seasons 2 and 3, I don’t think any other villain would have had such dangerous consequences for the entire series in the long run.