Dear Princess Celestia,
What if I died? I've been on many a mission which could've ultimately resulted in my death. What would happen to the other bearers of harmony? What would my friends feel like? How would Spike cope? How would you feel?
Your Faithful Student,
Twilight Sparkle
Dear Twilight Sparkle,
It has been many a month since your last question letter, and I was hoping for it to be not as morbid as this. Your death would severely hinder Equestria's scientific progress. You were the most powerful unicorn in Equestria, and will be for many a century, now you are simply the most powerful magician in Equestria. Anyway, now you're a princess I would expect you to be busy, but you don't need to fear about death. Hitmares are only in the old stories, where the land was corrupted. You're totally safe with your guards anyway.
If such did happen, which it won't, they would go into a long term of grieving, and Equestria would most likely crumble. Threats are always around, and without you, they can't stop the big guns. I feel that your second and third question are the same, so I'll keep this short. Your friends won't be able to do anything and they would go into a lifetime of paranoia.
Spike would not cope at all for the first hundred years of your death. Dragons live a long time, and to have his feelings destroyed at such a young age would devastate his future. Instead of a jolly, bouncy dragon such as he, there would be a lonely, emotionless being. I even see a chance of suicide, but that is unlikely.
I don't like to talk about my feelings very much but I know that I would feel devastated too. I am Equestria, thus I would crumble. An eternity of mourning upon a field of a burning Equestria would be all that's left - if you were assassinated or killed in battle. If you were not transformed, your death of age would bring much mourning, but not so much devastation.
Princess Celestia.
P.S Don't send any more morbid ones. I can't spend all my money on tissues.