Pinkie Pie = Luffy

Pinkie breaks the 4th wall a ton and has some very rubber-like bodily movements from time to time. She also can rush head first into a situation without any regard for her safety.

Rarity = Nami

They are both pretty girls who obsess over the finer things like fashion and money. While probably not the most effective in battle, they could hold their own when push comes to shove.

Rainbow Dash = Zoro

They are badass and they provide the strength and expertise in battle whether its swordsmanship or flying. They also have an expert they look up to (Spitfire - Mihawk). Also argues a ton with Applejack/Sanji

Applejack = Sanji

Sanji can cook, Applejack can prepare anything apples. While Applejack doesn't chase after stallions like Sanji does women, they both have super-powered legs. Also has heated arguments with Rainbow Dash/Zoro

Fluttershy = Usopp

The coward of the group but will ultimately pull out all the stops to help their friends if they REALLY have to. Can totally see Fluttershy taking on a masked persona to overcome the fear of a terrible situation.

Spike = Chopper

The little guy. Spike grew to monster proportions in Secret of My Excess like how Chopper can grow into his monster point form. They always look to aid their friends whether its to do tasks or to mend wounds.

Twilight Sparkle = Robin

Lovers of books, both looking to research mystic things (magic/poneglyphs).

Big Macintosh = Franky

Big Mac could have a tragic past that involves no parents like Franky but really they're just big dudes who are known to wearing the color red.

Vinyl Scratch = Brook

Well Vinyl Scratch is probably the most musically inclined pony in the show except she hasn't spoken at all. And she's probably not a total perv like Brook.....or so we think.

Yeah that's about it. I could do other characters but without the explanations.