rant shall start now.
-they shouldn't be an exact copy of an cannon character,think,brainstorm,be creative!they can have some similarity's (EX.England's former colonies and there eyebrows)
-there personality mostly is a reminder of there background or in hetalias case stereotype, do some research on the country if it helps.also think about who they were influenced by.
-your oc can not be a child of boyXboy,not hating on yaoi fans but just saying,even in anime it's not possible to have a child without a boy and female. boy + girl = baby!
girl + girl = NEIN!,boy + boy= NEIN!
-ALSO!TO ADD TO THAT in hetalia countries seem just to appear not