Having amassed the complete sets of the Sketchbooks and Complete Comics, you now need to gather up all the stuff that is:
a) Not included in the Complete Comics
b) Done after the Complete Comics (i.e. after about 1992)
Why not start with the 4 volumes of WAITING FOR FOOD? These are collections of drawings done on restaurant placemats while...waiting for food. Published by Drawn & Quarterly and Oog & Blik.
May already be out of print by the time you read this...