I speculate either an authentic set of retractable Kerrigan wings or a Kerrigan wig made from tree bark or painted bamboo (attachable to pets and chairs, of course).
Oh, here we go: An Overlord hat. The 'Sac' portion could go on your head, with the tendrils coming down. You could even fill the Creep sacs on the side with a beverage of your choice! Plus, it will feature 'metamorphosis mode,' because it is completely reversible. You could turn it inside out and make it an Overseer!
What would you all like to see in the next Collector's Edition? Discuss!