If Mexico was to be a character in Hetalia, I think there'd need to be TWO of it. Because see, the first one would be New Spain. New Spain would be a female character, bronze skin and black hair and dark eyes, a native from the country before it was conquered who then was forced into a marriage with Spain. New Spain doesn't understand Spanish and Spain doesn't understand the dialects, but still they remained 'married' for over three centuries.
And then Mexico is born, half Spaniard and Half Mexican, bronze skin and green eyes. It'd be both a very loud character and very respectful. Mexico would be constantly angry with his 'dad' and he'd be totally a mommy's boy, because New Spain is still alive, just that she's old now, gray and dying, but happy and proud of her son.
I've been also thinking about the encarnation of the FICTIONAL countries. Code Geass' Britannia looks a lot like V.V., but she's white haired and much more yandere. Tales of Symphonia's Meltokio is an androgynous looking character, snobby but paranoic. KKM's Shin Makoku? That already exists, and is called the Shinou aka Murata's shoulder demon.