A Power Ranger. Any Power Ranger. At all. Shut up, they're technically game characters.
Companion Cube, or Weighted Storage Cube. GLaDOS, even.
Megaman.
Misty from Pokemon. She's overdone. I wouldn't mind seeing someone cosplaying Brock though.
Anyone from Red Alert 3. I will hunt them down if they even try.
Kane from the Command & Conquer series. The only person who should even be allowed to cosplay Kane is the guy who plays Kane. If you're not Joe Kucan, you're committing a sacrilegious crime and you deserve to get shot in the face because of it.
There are others, but they've already been mentioned.