A small "what-if" world where the more Pony race's existed in Equestria, and none of the main six share the same species. While Twilight, Rainbow Dash, and Applejack keep their original species, Pinkie, Fluttershy and Rarity's are different:

Pinkie Pie is a Sea-pony:

Fluttershy is a Bat-pony:

Rarity is a Breezie(Breezie's are the same size as normal ponies in this universe):

2539555 Races doesn't have an apostrophe.

2539555
It could work, and seems mildly different than what is the norm for stories on here. If you write it well enough it could be a decent world building, show-styled episodic tale, depending on what you want to write.

I for one, spend time laying in the darkness of my room and on the occasion think of a new race that is a slight offshoot of normal ponies (like how crystal ponies are slight alterations to earth ponies) and so on. As of yet I have a few of my own original ideas floating about. I find it much more satisfying personally but if your aim is to expand on these races or simply that you find them interesting in your opinion, go for it.

A reoccurring problem here would be Pinkie as a seapony though. If she has to remain in water, how would she travel to certain places with the rest of her friends?

2539555 Nothing says that they're aren't Flutterponies with horns

2539555
Is it bad that when I saw that picture of sea-Pinkie, the first thing I thought of was shrimp?

2539620

I like to think that maybe Sea-ponies had an ability to manipulate water with a type of magic, giving them the ability to create a water sphere that they can get into and move about on land.

2540804
Well there you go, that's a reasonable idea and at the same time pretty creative.

2540804 Given that Pinkie takes physics as a mere suggestion I could see her swimming through solid ground.

2541584 I could see her swimming in people's eyes, over a rainbow, and through the moon. She's just plain batty. Er, no offence, Flutterbat.

2539555 it isn't as if Sea-Ponies are new.
Flutter shy was the Bat Pony and Rarity was a Breezie for a short while.
Just that there is also the Flutter Pony to look back at, if we are to see what was.
Just as the small detail, there was a very different Breezie in the past too.