Last winter, Tapastic had a community collaboration where characters from various series crossed over in a Winter Street Festival, which was a fantastic idea.
With Halloween fast approaching (and myself running a Halloween themed comic series) it would be fantastic to find a theme or setting for other creators to crossover / collab their own characters across.
Perhaps we can do a secret Santa sort of deal, where everybody draws each others characters, or maybe all the characters are trapped in the same haunted house. It's mostly open for ideas right now!