(T>B) The Twilight Era · 4:46pm Nov 21st, 2020
My entry in someone else's thread: https://www.fimfiction.net/group/50/the-writers-group/thread/419194/the-twilight-era-your-ideas-about-princess-twilights-reign-over-equestria
I started by concurring with someone else's suggestions of technological advancement, more Schools of Friendship and a vault for dangerous magic. Then I put
This probably lays out the key areas - Princess of Friendship is a job for life, and I also see her as a moderniser. In fact,, I could see her pulling an "Applejack's Day Off" on all of Equestria, to the point where she needs her friends to remind her to take things more slowly unless she wants to unsettle ponies. (If she didn't have good advisors like that... well, it's interesting that this thread was posted so close to one on "Stalinisation of Equestria".)
Her tech strategies will also be inherently shaped her her access to "advanced previews" from another world - as well as knowledge of all their mistakes. I can totally see a top-secret "Project Looking Glass" with the job of analyzing human technology, guiding Equestrian scientists towards (or sometimes, away from) it, and developing strategies to prevent or mitigate resultant environmental or social damage.
One thing that's canon, of course, is a strengthening of relationships with other races, within Equestria and out - cows may finally get full citizenship, for instance. A possible result of this is Equestria moving from a position of international neutrality to a more interventionist one, like America in the 20th century. She might even establish formal diplomacy through the mirror, though that might endanger Looking Glass.
And finally, I can see her returning to the mystery of alicorns, perhaps in search for a way to not lose her friends so soon. Perhaps nothing will come of it, or perhaps in a century all ponies will be alicorns - I suspect a truth in the middle, with a few becoming a few dozen,
And that might make the age a brief one, for eventually the Crown will no longer be able to absorb them all - a new system of government will be needed. Don't worry though - even if she wasn't Princess for more than a mortal lifetime or so, she still accomplished a great deal. And I'm sure she'll do more in her several terms as President (if alicorns are allowed to stand).
