What vegetables hold up well for making vegetarian pot pie?
What do you mean by hold up well? Structurally? Carrots and peas are rather common in a traditional chicken pot pie. Cauliflower is probably way too delicate to hold up well, also it's too mildly flavored to serve as a primary flavor. 

I would suggest trying some heartier root vegetables or starches. Potatoes, squash, zucchini are some good possibilities. I'd also add some onion and celery for a better flavor punch.