Holland has appeared. Only a
couple official pictures so far and a few official comics, mostly about Netherlands, America and the opening of Japan, which are in the main Hetalia LJ comm under the "official site" or "netherlands" tag.
We're at the point where the series has been around so long that
everyone is cosplayed. Except Cameroon, since we only have FIFA images of him. Oh, and a couple micronations were super-recently introduced, (Principality of Wy, Principality of Seborga) so they probably don't have cosplayers yet. If you want to do a rare cosplay but can't choose a rare character, doing a historical or traditionally clothed version might work. The outfits drawn for Children's Day are pretty rarely done, (except Austria and Hungary) and the official child versions (They're different things. It's confusing.) of most of the characters are also pretty rare. If you want to do some research and come up with a historical costume for a particular nation that isn't used officially anywhere, that'd be pretty awesome.
That is some seriously awesome skill right there.