you could have it twards the end, after zim's ship gets blowen up or something he could be drifting in space in the little invisable bubble hemet space suit thing and the in the distance he sees the santa suit, and realises he can reprogram it to anialate the humans, starting with dib.
the mission would be simply to distroy the santa suit, and all you'd need is a fairly bad looking blob in the general shape of santa and make the candy cane arms and spider legs as turrets, when you'r close enough they'll swing tring to point at you wich will result in you getting smahed around, when you're further away, they can shoot some sort of candy cain missles at you.
you manage to blast it's head off or something (spilling out ZIM) "but santa lives on"...

Zylon you havn't seen the Zim Most Horrible Xmas Ever christmass spectacular!?
you must get it, NOW!!! it's one of, if not the best, Zim ever, especaly the ending
...yes 2 million years