Design details pasted from the source with minor formatting improvements:

> After as discussion about Twilights new design, and how celestia’s body type didn’t look good on her, I decided to sketch my own take. So imbued with “ruler of equestria” levels of Alicorn magic, I believe that twilight would have grown in height and gained a flowing mane.
>
> However, with how “ribbony” her hair is usually drawn, I didn’t like how it was handled in the finale. I think it would look more like a flowing version of her normal mane. A bit less motion, and keeping the ribbon sections intact.
>
> I also think with the height change, she wouldn’t be so tall as celestia, but more likely Luna or Cadence’s height. There was also the discussion of her being a mortal alicorn so she wouldn’t have to watch her friends die off.
>
> With that in mind, I wanted to add some hints of age. She has gained silver streaks in her hair, and sometimes wears reading glasses when she needs to do long nights of research so that late night book reading is easier on her eyes. I also gave her a curved horn, because I think it is a good look on someone with powerful magic. I also made her crown and necklace silver to match the streaks in her hair. This is just a personal choice though.
>
> I hope you all enjoy!

There are more details at the source about how this piece was drawn.