What is the best ingredient to add to a normal French toast recipe to make it taste pumpkin flavored?
Not meaning this as a snippy answer at all, but I would say: pumpkin.

I would suggest substitute pumpkin bread and continue as normal.  This one I have done before.  It is a heavier French Toast (or eggy bread, pain perdu or many other names around the world), but turned out quite nice in my opinion.  

Another option would be a stuffed version with pumpkin as the stuffing.  And also coming to mind would be a pumpkin butter or compote topping on a recipe you already use.