hm...the possabilites:
a shaving from the Goblet of Fire
a shaving from Mad-Eye Moodys wooden leg and fake eye

a piece of the golden egg from the second task
a piece of the daily prophet rita skeeter article (common, gotta continue the Daily Prophet cards)
A galleon card #'d to 1000 (since that is the prize money for winning the tournament)
a piece of the beard the Weasley twins sprout when they try to put their name in the Goblet of Fire
Irish and Bulgarian team quidditch robes from the world quidditch cup
A leaf from the headges from the last task

#'d to...oh, I don't know, if the trailer is any indication, 10000000

Swatches of the tents the weasleys, Hermione and Harry stay in for the Quidditch World Cup
A Dumbledore, Snape and McGonagall costume card...FINALLY
And the list can no doubt go on and on and on for ideas.