Yes do a theme!! I love fancy dress it's so much fun, I find the less common themes are usually better. Nurses, bunnies and army gear can get a bit boring. The best fancy dress I've done so far was superhero night where everyone had to come up with their own superhero name and costume. My customised cape was all sparkly and amazing =D. Got a mighty boosh fancy dress in Feb, think I'm going to go as Naboo to make use of the cape again =p.
But yes fancy dress is fun but you def need a theme, and the more customising you can do for your outfit the more fun it'll be!