Find an item or some costume piece like a hat, prop, or mask and use it as the bases for a character. Let your imagination fill in the gaps.
We made some foam latex masks last year for our haunt and to sell that were a little different, and they made the bases of some great characters this year. We used one as the bases of a character called Gourdo The Pumpkin Patch Pirate. He was an excellent cue line character cause people didn't know what to make of him. People love to attach identities to characters 'Hey, it's that guy from that movie... blah, blah, blah'. It makes them more comfortable if they know the character. If you can create characters that bare little or no resemblance to anything they may identify it throws them off. They don't know what to expect out of the character.
Check out our masks at http://www.wickadella.com/masks.html