Okay, so, here's another story idea I came up with, I look forward to hearing your thoughts!
Almost all ponies mentioned are real, all those who become 'seasons' are actually canon,(Except Winter) look it up!
A long time ago, in the magical land of Equestria...
"Princess Luna, are you sure this is a good idea?" Fluttershy asks the last remaining princess.
"It's the only hope of Equestria, four of our daughters shall be brought forth to Royalty, and create the new balance." Luna replies, then glances back once. "And please, call me Star."
Perhaps it is best to explain, the world of Equestria was thrown into disarray when three of the Princesses, Twilight, Cadence, and Celestia, we're discovered missing. Soon enough Celestia and Twilight were discovered dead.
They found Cadence alive, but she wasn't... the same.
Luna was hopeful however, and the tried to move past it, Shining Armor aiding his wife with her newly discovered night terrors, and Luna ruling Equestria with the help of the rest of the mane six, as well as Starlight Glimmer.
A month passed uneventful, but all the Mane six except Fluttershy vanished, soon found dead once more, investigating the bodies led them to find that the deaths were not natural, they were magical.
Cadence began having even more sickening dreams, until finally, she realized, it was her. Not truly her, but she had been possessed, she left a note to Shining Armor before committing suicide.
Devastated, Shining Armor was the perfect next are for the being, however Luna and Fluttershy were able to contain him, but the being was part of a race, and the race condiment this an act of war.
Now the only way out of this destruction, is to remake Equestria.
They have chosen four ponies to become alicorns, stewards of the seasons, and now there shall be five kingdoms.
Summer, Fall, Winter, Spring, and Star.
The Seasonal Kingdoms shall be ruled by the kingdom of Star, witch guides them when one is suffering.
Fluttershy was chosen to become Princess Spring.
Cherry Berry was chosen to become Princess Summer.
Golden Harvest was chosen to become Princess Fall.
Nightingale was chosen to become Princess Winter.
Luna became Princess Star.
When they became alicorns, their appearance changed, as did that of all ponies in Equestria.
Princess Winter changed her entire appearance, her eyes became periwinkle, her mane a bright turquoise that seemed to melt into constantly falling snowflakes at the end, yet it never left a trail, her coat became a bright pale blue, her wings were made of ice and her cutiemark was that of an intricate snowflake. She also gained height.
Princess Spring didn't change too much, she became as tall as Luna, her mane grew out and curled at the end, also adding streaks of green that curled like vines, her wings became those of a butterfly. Her cutiemark was still three butterflies, only there were vines curling in and around them.
Princess Fall became taller and turned into a pale orange pony with a red-yellow mane. Her eyes were green, her wings were made of giant maple leaves, and her cutiemark was falling leaves.
Princess Summer became taller, bright yellow, her mane a leaf green, as well as her eyes, her wings were feathery, like a bird's and her cutiemark was a sun and a leaf.
Princess Star stayed pretty much the same, only with an added height bonus(boosting her to Celestia's height), and an added star on her cutiemark.
"Now, Prin- Lun- Star." Spring looks worried. "Will we be immortal?"
"Yes." Star replies. "But you will have to allow an heir to take your place after so long, only I can remain ruler."
"Why?" Summer inquires.
"Because I am the only true Alicorn here." Star responds.
"I see! Okie!" Fall smiles brightly, causing the other seasons to roll their eyes. Star opens a vile.
"Put a small amount of your magical essence in here, in a time of great need, this will aid us, just as Luna, Celestia, Cadence, and Twilight's magic aided us before." As per tradition, there is vile kept hidden and safe that contains the magic of the first rulers of a land, it turned Equestria into the Four Kingdoms, and it will aid the Four Kingdoms in the future.
All the seasons follow, adding a small amount of their magic, then, they fly off to their new kingdoms, Star encloses the vile in a chest inside a vault, the she locks it with her magic, and flies to her new future, not looking back.
Okay, so I mentioned the normal ponies changing too, they do.
In the Kingdom of Spring:
Unicorns: don't change much, but they tend to have bright colours.
Pegasi: have insect wings (Except butterfly or moth, those are only allowed to the royal family)
Earth ponies: also basically just have bright colours.
In the Kingdom of Summer:
Unicorns: Have living branches for horns.
Pegasi: have green leaves for wings.
Earth Ponies: don't change, except for brighter colours.
In the Kingdom of Fall:
Unicorns: Have dead branches for horns
Pegasi: have fall coloured leaves for wings
Earth Ponies: Fall. Colours.
In the Kingdom of Winter:
Unicorns: Icicles for horns.
Pegasi: basically they have snowflakes for wings, each family has a pattern.
Earth Ponies: pale blues, whites, and silvers.
The ponies in the Kingdom of Star just become darker colours, navy blues, purple, blacks, stuff like that, the Unicorns also have starts on their horns, and pegasi sings seem to be tipped with Stardust.
Bye!