A Space Theme could be fun - some sort of sci fi theme. A friend of mine is having a May The Fourth party - it will be Star Wars themed. There will be lots of Princess Leia type slave girls, complete with decorative collars, chains, and tiny bikinis... unlike Leia they will probably be lots topless. His GF just had a boob job so I'm willing to bet 100 Imperial credits she'll be collared, chained, decoratively dressed a bit, and topless.
Roman themes are always fun. Lots of fun togas and slaves and baths. Watch the show 'Spartacus' on Skinamax for inspiration.
There is a party coming up with a Alice in Wonderland Theme. That should be fun. I have an interesting Mad Hatter sort of costume I could wear.
Medical themes... Easy to get some scrubs, some speculums, and walk around saying you are a 'freelance proctologist'. Also, instructions on how to perform a prostate exam are available online. Probably.