Back when the Satanic Ritual Abuse bullshit was going real strong, I went out as a Child-Abusing Satanic Cultist in one of those nylon Grim Reaper robes, with an inverted cross around my neck…and carrying a nekkid baby doll skewered on the point of a plastic sword. I kept offering onlookers a taste of my home-grown and raised devil baby “Hey, it’s organic – we breed our own!”.
Another year I made a humungous witchdoctor mask out of cardboard, with horns and everything; the rest of the costume consisted of a leotard, tights, grass skirt and a papier-mache skull rattle. That one pissed off several different random strangers.
5 or 6 years ago I went as a suicide bomber. I made an explosive vest out of an old load-bearing vest and some PVC piping, wrapped a shemagh around my head, threw on some sandals and off I went. Upon arriving at the evening’s party, I yelled “BOOM!” Fortunately my friends are as twisted as I am.
Dress your top half in army gear and you lower half in sports gear, find yourself a wheelchair or stuff you left or right arm inside your sleeve and sew shut, garb yourself a white A4 piece of paper and write in black a number on it, tag it to your front or back and tell people you have started training for the 2010 Special Olympics knowing that with the war still going on the competition is going to be stiff this year…(courtesy of Jimmy Carr)
Not relevant, but years ago someone on this board went as Jesus Fucking Christ. He dressed as Jesus and had a blow-up doll also dressed as Jesus bent over and attached to his groinal region.
Good one. 
A friend of mine once dressed in a pink skirt splattered with red paint and a blond wig. She carried a baby doll and a coat hanger, also splattered with red paint. She was Do-It-Yourself Abortion Barbie. Offended a lot of people, as I recall.
Go as the internet. Attach a lot of toilet paper tubes and paper towel tubes to yourself, roll up pages from porn magazines, advertisements etc. (or print out stuff), and stick them in the tubes.
Bonus points for including pictures of typical internet memes. See how many your friends “get”.
You should get your pals in on the gag. Everyone’s Jesus but someone has a pogo stick, someone else a giant cracker…possibilities are endless!
How about dressing up as God and painting your face black and go as Barack Obama?