I cosplay Fem!America. The one Hetalia event I went to was Hetalia Day a couple years ago.
The image I've always want to do was Beautiful World
. Personally I'd like to do Hungary or Taiwan, but I think we could make a fun get-together out of it. Everyone shows up with a white t-shirt and the appropriate bottom depending on their character and we can decorate the shirts accordingly. Could be at a picnic ground or something where we can have food and games to go along with it (give time for the shirts to dry). Then take the photo at the end.
Edit: This could also carry over to other conventions where we can recruit missing countries (even those not in the image) and use it as a nice group photo.
I can also crop certain characters that people are interested in so they can see the design better).