Or maybe Turnip Age...
Anyways, here's a character whos design I just assed together in like half a second, but ok so this is for my fanfic ahem
Twilicorn has been getting more powerful and Alicorny but has to do a lot of missions away from ponyville for long periods of time. She takes note of a young pony much liek her called Turna Page who spends a lot of time in the Canterlot Library. She offers Turna the position of Ponyville Librarian while Twi is all busy and such.
So Turna becomes the new ponyville librarian and the Mane 5 like her a lot and become friends. But as Twilight returns later she finds that they are like super great friends, and while Twilight is still friends with them she has grown distant from the group due to her extracirricular princess studies! She's missing out on all the in jokes and small town adventures.
So Twilight begins to become distracted, angsty and most of all devilishly envious of this Turna Pony, what with her no responsibilities and her carefree ways, and begins to cause small havoc's on her missions. She is confronted by Celestia andf Turns against her! "How could you show me this friendship, and then begin to slowly tear it away!?" After an intense battle she sucessfully imprisons like Celly Luna Cadence Shining and whoever else.
So this part is maybe idk, but in the spirit of chaos twilight could become some mental distortion of herself, basically becoming a stone cold ruler who imposes strict library type order on all of ponyville, and NO FRIENDSHIP but idk.
So she goes to direct this angst at Turna but, at the last moment realises what she is doing, to this young shy pony much like herself just discovering the magic of friendship. And she understands why Celestia taught her this, for while friends fade, the feeling of friendship and its importance will always linger. So she restores order and goes into hiding somewhere.
But then, Turna leads the hunt for hiding Twilicorn, leading all the mane 5 to find her, because on the flipside she understands from her life prior in relation, how it is to not have friends. So they find her and she tries to keep them out with magic but Turna wins her way in with admiration.
And then Celly and Luna show up and tell Twilight she is still young and there is much time, but this was a taste of her future responsibilities and to never loose sight. They take away her powers for now so she can go on being a normal unicorn and turna stays too and they all have great fun!
So that's how I'd do a movie between seasons 3 and 4.