As the title says, au in which the 2P character types are instinctual sides of the hetalia cast which come out during times of great stress and danger. Perhaps Russia is like how he is because his nation was almost constantly in danger as he developed, therefore making that side of him a constant past of him? Italy being note to beat Turkey because of that other side to him?

Bonus if you include Prussia with a 2P character, whatever you feel they'd be like, or other countries that don't have established 2Pmcharacters, couch as Finland, Sweden, Austria, etc.

Although the pairings are up to the filler, one of my favourite pairings include Prussia/Austria.