you could do an initial party - like LC suggested - have everyone come as something starting with one of your initials (first and last or just first etc)
i went to a 'pimps and hoes party' once
lol that was really fun though i've got to admit!
hippy party? not a decade but lots fun
fluro party - everyone has to come in fluoro colours
crazy dress party - everyone should wear crazy/mismatched/weird things - also a lot of fun, normally very bright!
colour party - my friend had a black and white party, everyone had to wear black and white and she wore yellow to stand out (or you could wear black and white) works with any colour though!
movie party - pick your favourite movie and theme it up - mine would be breakfast at tiffany's, something glamorous, can work with any movie really
crazy hat party - everyone has to wear crazy hats
that's all i got for now
hope that helps