WELL, see, here’s the issue. Since I’ve seen the S7 finale, here’s the issue we’re facing. Oh yeah, and spoiler alert, I suppose. This is already known info though.
Sunset Shimmer isn’t CONFIRMED canon. However, she does EXIST.
Really weird, I know.
To put it simply, the sirens are canon because Starswirl and the others banished them into what he believed was limbo.
In truth, they banished the sirens to the human universe.
This means that while Sunset Shimmer does EXIST, there’s no area we can place Twilight discovering the human universe, or the mirror, within the current timeline without causing some issues. Major ret-cons and all that. For example:
Why is there not anything explaining how Twlight can artificially open the portal?
How does Twilight move the portal effectively but seemingly has no concrete space for it? We see in the specials that it’s in the library, but then it’s gone from the show.
Why isn’t Sunset making a more prominent part of the show, such as PERHAPS apologizing to Celestia?
Why not collect data from the human universe regarding technology that can be used to improve things in Equestria?
The issue here resides in the fact that there’s so much substance that CAN be developed, but apparently isn’t.
