Well, this is a picture from last Halloween (my costume wasn't completed yet though).

Ok I am not the best judge of this (far from it) since I think anyone should be able to cosplay as who they want, so bear that in mind. Also, this would be easier if I knew what characters people like.
Calico, sorry I'm drawing a bit of a blank on specific characters.
Venus-Castina, I think you could make a good Integra Hellsing if you wanted to.
Jennifer, I think you could do Amy Pond from Doctor Who.
There are several other characters you could cosplay as (depending on resources), but I am drawing a bit of a blank at the moment. I also firmly believe you could cosplay as whoever you want.
Edit: I like villains. The chances of me playing a hero aren't very likely unless they're creepy.