Why. . . ? QAQ  
Side note: Don’t people normally associate Applejack as Sanji, because of her strong kicking, and wouldn’t it make more sense to have one of Pinkies actual sisters as Ace?

Nah no pony in MLP is hard enough to be Akainu.

So, whos Akainu?  
Cause Im voting Blueblood.

WARNING: Spoilers regarding One Piece follow.
 
 
I went with AJ and Pinkie Pie rather than AJ and AB mainly because Ace isn’t actually Luffy’s biological brother. So using biological siblings to play the part of non-biological siblings didn’t work for me. And yes, there also was the issue of character, in that Pinkie is so totally a Luffy-type character.
 
I chose AJ to play Ace because she’s such a hot-blooded, strong-willed individual who cares deeply about people she considers friends and family, and being the Element of Honesty, she’s going to speak her mind, just like Ace would.
 
Fun fact. I was going to make Dash be Ace instead of AJ, until I decided that I wanted Dash to play the part of Franky. You are now imagining Dashie as a huge cyborg with rainbow Ace Ventura hair.

Or because AB is too grounded to be Luffy.

Shouldn’t it be Apple Bloom instead of Pinkie?  
I suppose the artist went with the “rubber” connection rather than the sibling angle.

Spoilers: Snape Kills Ace