With Marvel and DC expected to share another joint venture this year, I feel it's only right that the wonderfully creative fans here at CBR get a chance to chime in on who they think would make great additions to the amalgam universe. Feel free to create new mash-ups or take existing amalgams and tweak them to your hearts content.
And don't forget that both publishers have a number of bought-out IP under their banner (e.g., Malibu, CrossGen) that rarely get to see the light of day. Feel free to use any character used by either company, even if the character has more or less been mothballed.