Scott Sonneborn takes over as showrunner.

Daybreaker

@Background Pony #4865  
zedd would

@Background Pony #4663  
This is one of the greatest answers and now that power rangers is Hasbro we could even use this head Canon and say it’s Lord zedd crossing over my 2 favorite Hasbro properties of course there is no reason I can think of that sees would turn equestria to dust so probably not

My headcanon is that it is the work of a villain from a different Hasbro property.
 
Megatron would strip the world bare for energon.  
COBRA would wreck the place with all the betrayals after the conquest.  
And Nicol Bolas canonically does not distinguish between his servants and his victims.

All the bads battled it out until there was nothing left.

@Background Pony #380D  
Interesting idea

My Opinion is that the Friendship Games happened except Sci-Twilight succeeded in total Apocalypse in absorbing magic from all Equestia and then laid waste to all before her. Think what happened when Raistlin Majere killed the gods in the Dragonlance series.

@reversalmushroom  
They actually won in two of their appearances. As far as I’m concerned they have the most villain cred of anyone here.

Okay, that Flim Flam timeline was really stupid that they (the writer) even put that in there.

The most grim possibility is the 7th one is the Cancellation Timeline, where all that would have been the future is reduced to a wasteland, any speck of joy and life removed to jam into the next generation of MLP.
 
The Hasno Timeline. The thought of it is indeed more terrible than anything else I could imagine.

“You crawl unwitting like a blind, writhing worm towards endless madness and despair. I have witnessed the True End Time! This?! This is a Blessing you simply cannot comprehend!”
 
-Arcana Empress Midnight Twinkle (formerly Princess Twilight Sparkle)

@Yet_One_More_Idiot  
Interesting idea, wouldn’t mind that being the case

@Detsella Morningdew
 
@Darth Shy
 
My headcanon is that the potential cause of the 7th timeline hasn’t even come to pass yet.
 
There’s something still to come. Something worse. Much much WORSE. Than anything yet seen.
 
And it will feature in the series grand finale.

My headcannon is Daybreaker could be the 7th timeline

@Detsella Morningdew  
Oh crap, that actually makes sense

My headcanon is that this is the timeline where Starlight succeeds far more than she hoped - where nopony messes with her village, and she manages to take the Eternal Sisters’ cutie marks.
 
What happens when you take away the power to move the Sun and Moon, and no unicorns have been trained to do so for thousands of years.

Let’s be fair, we all know what happened here.  

At this point, I’m guessing it was the Pony Of Shadows.

@Dirty Bit  
No, not that Smooze (which we shall henceforth never speak of again :P) …the real/original one. (So, the Witches from the Volcano of DOOM)

@Yet_One_More_Idiot
 
Then it’d be Discord’s fault, no?

A WILD THEORY APPEARS!
 

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I’ve heard that nothing can stop it…and it’s gonna get to you. And you. AND YOOOOU! xD

There is another possible answer to who caused the ashlands timeline…  

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**It’s none other than the King of Storms!**

“It is finished when seven are one”

Daybreaker