My opinions:
Ethnicities (not to be racist):
Twilight Sparkle - Afro-Latina (just like Miles Morales from the Spider-Man series), thick black hair with bangs, dark brown eyes and glasses.
Pinkie Pie - White, curly hair with magenta hair dye and blue eyes.
Rainbow Dash - White, thick blonde hair with rainbow hair dye and pale blue eyes.
Fluttershy - White, light brown hair with light pink hair dye and greenish blue eyes.
Rarity - White, dark brown hair and dark blue eyes.
Applejack - White with a bit of tanned skin, medium blonde hair and green eyes.
Scootaloo - White, light brown hair and light green eyes.
Sweetie Belle - Caucasian (as Rarity, but raised in Canada), long and curly pink blonde hair and pale green eyes.
Applebloom - White (no tan skin), long and straight red hair and yellowish orange eyes.
Nationalities/Origins:
Twilight Sparkle - American (with decent from Mexico, Puerto Rico and South Africa) raised in Miami, Florida, but moved to Atlanta, Georgia.
Pinkie Pie - Canadian-American raised in Detroit, Michigan, but moved to Bismarck, North Dakota.
Rainbow Dash - American (with decent from Ireland) raised in Boston, Massachusetts.
Fluttershy - American (with decent from Japan) raised in Las Vegas, Nevada.
Rarity - French-British (with decent from South Korea) raised in Manchester, England, but moved to Newark, New Jersey.
Applejack - American raised in Dallas, Texas.
Scootaloo - American raised in Springfield, Massachusetts.
Sweetie Belle - British-American raised in Newark, New Jersey.
Applebloom - American raised in Dallas, Texas.