I think I found loop in the Ninjago's timeline or whatever. Who remembers the Great Devourer from season one? Maybe you did. I don't know because honestly, we've got Heads of Time and a whole movie to be excited about!
Back to the theory, how was the Devourer defeated? The four golden weapons. Is there any other ways to defeat the Devourer? No, there isn't. Lord Garmadon says this himself in The Day of The Great Devourer. Which is also the last episode of that season. The only way is the four golden weapons.
Now let's skip ahead to -uh- (frantic typing) Wrong Place, Wrong Time! Episode 19 in the whole franchise. Also found in season two. The ninja travel back in time to stop Garmadon who's still evil (Come on, dude. You can't be the bad guy for two seasons!) from ruining the future. In their haste to stop that, the ninja actually did that for Garmadon. Here's why:
The ninja destroy the golden weapons The golden weapons are the only way to defeat The Devourer Hence, The Devourer in the past should have ate everything That makes it impossible for the future to still exist Hence, the canonicity of the rest of the show is questionable.
That doesn't mean it's not cannon, it's the fact that, it should not truly be cannon. It's impossible! There is only one way to hide and that was underground but, that only snakes would know that. Also, they'd all starve to death anyway. Not much huh?
Why are the ninja living then?
How is the rest of the show remotely cannon?
It's clearly not possible.
If it's clearly not possible, then really, everything after Wrong Place, Wrong Time should not happen. In fact, season 2 and onward shouldn't happen. The only exception (I believe) may be the Ninjago Movie. It's trailers so far, show that it's reimagining what happened in the beginning.
In all, nothing from season 1, onward cannot happen and then it makes the canonicity questionable. The only thing that is an exception may be the Ninjago Movie.
The research only took like a minute or two. Realizing the theory took a few years.. I started watching it when the first episode actually came out.. That was around..(researching).. 2011 says wiki and it's 2017 so simple math says it took me 6 years to realize this since I watched Ninjago… Yeah this took forever to come up with.