I haven't seen a thread about this yet.
I want to start this thread, when I saw that there was one last year. So if anyone planning to dress as a Key character lets gather.
I think I going to be Tomoya, and if I grow my hair out, I could be Sonohara with black hair.
I was Tomoyo (winter uniform) this year, and the only other Clannad character I saw was a Nagisa. If there's a fairly decent gathering this year, I may consider recycling my cosplay (or make the summer uniform in celebration of Tomoyo After).
Well, if I felt like being the laziest "cosplayer" ever, there's always Yukito (http://www.tbs.co.jp/anime/air/chara/image/yukito-b.gif). XD
With Key stuff, it really has to be girls cosplaying for there to be any effect.
Don't forget Little Busters. Though, since there's no anime (yet, at least), I doubt it'll see much/any cosplay.