The Mane Six - and most other ponies - have more than one talent. In another world, they could have had entirely different cutie marks. That doesn't mean they're not friends.
The POV of the five ponies who find the world is not the world they remember, nor the one their cutie mark is telling them. They were happy with their cutie mark before, what's changed? And how each of them got their cutie marks.
When the Great and Powerful Trixie manages to vanquish an Ursa Minor rampaging through Ponyville, Twilight Sparkle seeks revenge for her ruined reputation.
After a long time in hiding the truth, Twilight is ready to tell her friends the big news and bring her colt friend to light. Reactions are bound to ensue
Princess Nightmare Moon had tried to keep it secret. She had feigned ignorance, but the charade could not last. She nocked the arrow. Tirek was a large target; he would fall easily. Then the true challenge would come: explaining herself to Celestia.
Nightmare Moon won. She defeated Celestia, broke the Elements, banished the sun. This all happened a very long time ago. So long ago, that she has had plenty of time to change her mind. Not that fixing it will be simple.
Rainbow Dash was enjoying her new unlife. Partying all night, doing whatever, whenever she wanted. Her new gang, the blood pact, were more then happy to indulge the mare's lust for violence and mares until she gets a letter, delivered by Fluttershy.
500 years after Luna's banishment, Celestia in given a wishing stone. She uses it to wish for Luna to be with her angain. But there is a side effect of to this good fortune.
Instead of being absorbed by Tirek, Discord's magic shoots away and goes to the last pony anyone expected: Celestia. Now Equestria must cope with thier most experienced leader being a bit...stranger
Starlight returns to Canterlot to pick up her things from her home to bring to her new home in Ponyville. After she does this, she and her Canterlot friends meet to talk about what happened recently and about her starting to live in Ponyville.