i know what she's sad about (though i kinda making it a short story)
a couple years after she became alicorn fluttershy got hurt when she stopped king sombra (who returned from the shadow) and she might not make it. Then pinkie pie got really sad and isnt eating or sleeping**because im a total pinkieshy** and then Twiligh relizes that she will out live all of them. Forever.
its sad really but i love how you did this. its a true masterpeice
Twilight is sad that one day she will die and leave her mentor behind, as well as all she learned from books will go to waste, little she know that one day she will become Alicorn by unlocking magic of friendship.
Well, other disadvantage is that she will outlive her friends, but there is pain and joy in everything, Twilight will need to move forward and find new friends, while she keep those she lost in her memory.
And why Twilight not included, I know that we need to wait for season 4 to see how she deal with change in her live and how others will act whenever they encounter her ( Twilight: P...le...ase, there is no need for you to bow, really, I may be a princess, but I am still your dependable librarian ).
In my opinion Celestia and Luna have great backstory and good potential, but they are heavy wasted by doing nothing in big events or being useless, it is sad when we got Luna saying " did I miss anything " while she could easy find her place in fighting off army of changelings to cover mane 6 or distract chrysalis so that Twilight can free Cadance so they can nuke Chrysalis and Changelings out of the city, instead we got a very repetitive arrogance once again being doom of villain ( how can a queen send her guards outside leaving her enemiesin the same room unguarded, how can someone who feed on love and who used love to defeat Celestia just say that it is useless ).
Seriously, Luna and Celestia are wasted and they deserve some action in season 4.
As for Cadance, her problem is that she took major role in 2 big episodes, but she have no backstory and build up, in here case we need to learn more about Cadance past, in my head canon Cadance was Crystal Pony who created Crystal Heart and it turned her into Alicorn, she was choice as princess, then Sombra kicked her out and took over, Cadance asked Luna and Celestia to help, but Sombra took empire with him and so Cadance was stuck in Equestria, she was princess but no power so she just spread love and played with ilusion spell to make herself look young and cute ( I do not buy she was born 20 years before wedding episode, why would crystal ponies call her crystal princess then ? ).
Other backstory about Cadance is that she was pegasus who turned into Alicorn by spreading love and Chrysalis was evil witch who wanted her down or something like that, but we would really use Cadance do explain her backstory in season 4 as she already took a nice action alongside mane 6.
And we also have Twilight, she is already a hero and element bearers, so no need to change it, there are many ways she can go in her episodes as she practically start from level 1 once again but on hard mode.