Whatever you do decide on, take lots of pics. I'm not a vegetarian, but I would love to see what you come up with. The Wellington does sound like a presentation win, especially since you can make festive shapes with the dough.
I still think something veggie/fruit based served in that insane...
While soup may not be a traditional centerpiece, a butternut bisque (made with coconut milk) served in a roasted sweet pie pumpkin is not only incredibly tasty, but gorgeous as a centerpiece.
Use a braising pan, and pile up roasted Brussels sprouts around the base of the pumpkin, and you've got...