Are we talking an at least semi-nerdy audience, or a random crossection of people? Because that changes things. You should probably be something that's not too obscure or else the only person who appreciates the costume would be people who ask you about it. Also, it should be a character whose appearance is iconic and at least thematically consistent, so its recognizable. If a semi-nerdy audience, Id look at the zombie angle and consider Zoey from L4D. Dress up as her and anyone familiar with L4D will recognize her immediately, and it probably won't be too difficult to pull off. Or the Witch. If a less nerdy audience, Wonder Woman is good, but probably likely to be overdone. Jubilee from the old X Men animated series might be recognizable enough, and she is very distinctive. but honestly, the best choice probably is Carmen Sandiego: That is an inspired choice.
