Cat Nurses from Doctor Who, and me, smiling like a moron, DragonCon 2007
I have some costume advice for you. This isn't Doctor Who specific, but there are a lot of examples that I've seen that come from the Doctor Who universe. The advice is: never underestimate the appeal of recognizable side characters as a viable option!
For you, you don't have to compete for being the most realistic, best dressed or what-have-you of say, the Doctor Who's in attendance. For other fans, there is nothing like the warm thrill of vague snobbery when you recognize a character that "outsiders" (i.e. people unfamiliar with whatever canon you are) won't.
The Last Living Human Being, DragonCon 2007
That isn't to say that you should always dress as something obscure, or strange, but it can be really fun working on getting the details right. And it feels extra rewarding when people recognize you - it's like you share an understanding.
Dressing as side characters in a group is also a great way to make an impact - I have some friends who are dressing up as the Kyoshi warriors from Avatar: The Last Airbender for this year's Animazement. And I mean, c'mon, have you seen the literal army of stormtroopers that is the 501st Legion?
To end my rant about side characters and obscure costumes, I will post this piece of brilliance:
New Age Retro Hippie, DragonCon 2008
A costume of a random enemy called the New Age Retro Hippie from Earthbound (released in US 1995), a videogame for the SNES. Just awesome.