Your mainstream cosplays will always be a popular choice among the public(One Piece, Kuro, Naruto, Pokemon, Vocaloids, etc!). If your going as a minor character, then I suggest you should stick in a group(the bigger the better). Also you will see cosplays of recent animations like SAO, among with Korean MMORPG/Game cosplays.
Your right that you won't see any Hetalia here...
This upcoming Seoul Comic World will be on the last weekend, December 29 and 30. Im not sure if many people will cosplay though cause the weather is getting really freezing, but many students are on break now. You can always search previous events to find out the "flavor of the months" and soso.