Dear Twilight Sparkle,
Thank you for your detailed account of this voyage. My recovery goes well, it will still be a few days before I am back on my hooves in proper form.
On a much more personal note, I want you to know that I am eternally grateful to you, your friends, and Luna. If the Shadows had used my body to hurt anypony, I just don't know what I would have done. the thought is far too much to bare. I am indebted to all of you for your hard work, courage, and perseverance. Extend my royal invitation to this "doctor" friend of yours, I simply must meet him when not under the possession of those horrible monsters.
Viva has been appointed the personal pupil of Princess Luna. Their first order of business is the construction of the Royal Canterlot Lunar Republic History Museum. The building will be in honor of the thousand years of peace created by the Princess. It will be quite a sight, you will be among the first to be invited to the grand opening.
My dear Twilight, you are faced with a rare opportunity. I have known you ever since you were a filly. I know right now you are torn over what to do next. I would like to save you the trouble. I, Princess Celestia of the sun, appoint Twilight Sparkle to travel with The Doctor on his adventures, may she learn about the incredible wonders of the universe by experiencing them first hand. Go my little pony, see that which you never thought possible.
Your proud teacher,
P.S. I hear that this Machine of the Doctor's travels through time. If that's true, I expect the letter from your next adventure first thing tomorrow. Stay safe.
Thought I would provide a little bit more closure for you guys.
Thanks for taking the Journey with me
See you all on DOCTOR WHOOF: The Shattered Stone