The final version will have a lot more characters, but I figured I'd upload the first two I finished. And would you believe it - I'd been working on this since Sunday, and just this morning I see another (way better) artist had the same idea - she assigned a lot of characters to the same houses I did too. Hopefully people don't think I'm copying her.
Anyway, here's Grampa Rome and Germania. They're teachers at Hogwarts and the other nations are going to be the students. I put Rome in Slytherin and Germania in Gryffindor because I feel it reflects their personalities better. (But, but, but - colour schemes!) Yeah, no, I went by personality. Germania is a warrior nation; strong and stoic and very Gryffindor. Rome is defined by his ambition and people-skills (or "cunning" as the books might put it), so he's in Slytherin. Your mileage may vary, but this is my drawing so I'm using my mileage.
Hetalia (c) Hidekaz Himaruya
Harry Potter (c) JK Rowling
This fanart (c) moi