Someones idea of what the mane six should be in the mirrorverse:

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3579068 Not bad an idea but I am wondering what the world is like in general. Guess we writers have to work on it.

No, because there needs to be Flim and Flam as Fairness, AND there already IS a Mean Six, and whatever collection of heroes needs to include the members of the windows plus Discord, and there are no Elements of Harmony to wield.

I have been working on it, but my EG fic comes first. I have to finish it before Rainbow Rocks comes out.

3583506 But we don't know if there were elements.
Because discord was good they never went to see if there was a tree of harmony.
True their may not be or it may be some place else. Or they could be evil?
I just thought I point that out.

3590890 Celestia explicitly says that the Elements of Harmony DO NOT exist in his universe. That is why Sombra is counting on them to tip the balance. Which actually, they do.

The Tree of Harmony was there long before Discord had anything to do with it, so it wouldn't matter whether he were good or bad.

3590961 Well I was just saying that trying to find a way to beat Discord was what prompted the sisters to look for the tree in the first place and if he was good there would be no reason to look would there.

But now I am wondering why they don't exist.

3591401 That's also not true. The two sisters found the Tree of Harmony before they'd even heard of Discord. This is all in The Journal of The Two Sisters, which is about as canon as it gets.

Not everything in the Mirrorverse is exactly the same, only reversed. There's no Starswirl, either, because Sombra has only met one Starswirl.