SAO. The game had a world wide release and all the countries were trapped in the game with their people. Also it'd be interesting to see modern versions of the characters, I think most/all of Hetalia takes place over years 30 ago, but the world's changed quite a bit recently.
Hard choice, although it would be slightly interesting if it was crossed over with naruto, well it would also be kind of sad, because there would be alot of deaths. :/
Pokemon!!! Corny, I know but, gosh I tình yêu Pokemon and Hetalia crossovers so much! My most favourite childhood anime crossed over with my most favourite anime would just put a huge smile on my face! I would tình yêu it so much! <3